Date: Mon Aug  3, 1998

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Part 17

Part 17 ** This section is about paints and painting. It tries to answer some ** general questions about materials and techniques. Someday I ** hope to add a follow on section discussing various types of paints ** in more details. [Q] What are the dangers of painting? [A] (Don Schmitz 4/97) The obvious hazard in using any sort of paint is breathing paint overspray and solvent fumes. The solvents in most hobby/consumer paints are relatively mild and are used in very small quantities, but many of these chemicals have a cumulative effect (your body has a hard time eliminating them once absorbed), so that after years of misuse you may significantly raise your risk of cancer, liver disease, nerve damage, etc. Even with modern water based acrylic paints, some use alcohol as a solvent which isn't good to breath, and regardless of the solvent coating your lungs with actual paint is not a good idea either. The one painting safety measure everyone should take is to use an activated charcol respirator with filter canisters rated for "organic solvents" when painting. Such a respirator costs about $30 at hardware/home stores [got mine at Sears], and effectively removes most any petroleum/alcohol based solvent fumes. The filter canisters should have a fuzzy fiber pre-filter that does an equally good job at removing paint overspray. Once you've got a respirator, remember to replace the canisters (about $10-15) at least once a year, or sooner if you can start to smell paint with the mask on. The paper "nuisance masks" you'll find in the same aisle at the hardware store are worthless for paint protection. They're really only good for filtering out dust - they do nothing to filter out solvent fumes and I wouldn't trust them to do much for paint overspray either. Once you have the respirator, you'll need ventilation for your paint workspace (unless all you're using is water-based paints). When brush painting open a window and/or use a small fan to avoid having solvent fumes build up near your work space. For spray painting, proper ventilation means painting outside or having some sort of spray booth (see "What is a spray booth?" question in this FAQ section). Finally, make sure your painting area is no-where near a gas furnace/hot water heater. The pilot lights in these units have been known to ignite flammable liquids (such as paint thinners) spilled nearby. If you just *have* to use such a space, limit containers of flammable liquids to very small (less than 1oz) bottles, and refill them somewhere else. It couldn't hurt to have a fire extinguisher handy either. [Q] Why is painting such a black art? [A] (Don Schmitz 11/96) Painting tends to cause modelers a lot of grief. There are a lot of ways that applying a coat of paint can go wrong, and Murphy's Law suggests that the odds the paint will go wrong are in direct proportion to the number of hours spent building whatever it is you're painting. Worse yet, you will find numerous and conflicting opinions on the "best" materials and techniques to use that will solve all of your problems. Most of the difficulties associated with painting come down to there being a huge number of variables that can affect paint behavior, with many of them being difficult or impossible to control. Moreover, there are many different combinations of these variables that will give good results, and a much larger number that will give poor results. The problem with trying to duplicate someone else's successful technique is figuring out *exactly* what it is they're doing - they may well not know themselves. For example, they may say "I add a few drops of thinner X to paint Y before airbrushing to get a glossy finish" - which sounds like valuable information, but it doesn't tell you how big a "drop" is or how much paint to add it to, or what type of airbrush, size of airbrush tip, or air pressure. The modeler that it works for, of course, always uses the same bottle to mix in, same dropper, same airbrush setup, etc, so he never gives these variables any thought when telling you his secrets. I think you get the idea... About the best you can do is pick a paint family with the general properties you're looking for, and then experiment with it until you find a combination that works for you. Come up with a system that lets you do things repeatably, and then vary one thing at a time while painting lots of scrap plastic. Keep a notebook of what works well and what doesn't until you get a feel for how the paint behaves. Some suggestions for putting together your "system": - get a few of the same sized containers for mixing paint (get a lot if you go with disposable containers) and make a mark at the same spot on all of them so you can easily start with a known quantity of paint. - buy some disposable pipettes from a chemical supply store so that you can add precise amounts of thinner or other paints to your mix. - cut some sheet styrene into lots of .5 x 1 inch pieces, prep them the way you usually prepare your models for painting (eg. wash, sand and prime) to use as test pieces. Shoot a series of test pieces, varying one thing at a time (interesting variables are thinner ratio, distance from part, number of coats, airbrush pressure, airbursh needle size, etc). - when you actually paint a model, paint a few test pieces at the same time to test for dryness and compatibility with topcoats. [Q] What are all the different kinds of paint? How do they differ? [Q] What are these new "acrylic" paints I'm hearing about? [A] (Don Schmitz 11/96) Typical paints have 3 components: - pigment : the material (often a metal compound) that gives the paint its color. - vehicle : the solid material that is left on the surface after the paint dries. This is what gives the paint its protective properties. - solvent : the liquid that the "vehicle" is disolved in so as to make the paint liquid in the bottle. Paints are characterized by both the type of vehicle and solvent used, and also as one of "enamel" or "lacquer". An "enamel" is a paint that both "dries" and "cures". After applying an enamel, the solvent evaporates, leaving a layer of the vehicle material on the surface. The vehicle then "cures", undergoing a chemical change that makes it much harder and usually less soluble in the original solvent used in the paint. This is why it is so easy to finger print a fresh coat of enamel paint even though it is dry to the touch - it hasn't cured yet. A "lacquer" on the other hand only "dries" - as soon as the solvent evaporates the paint film is in its final state. Lacquers remain soluble in the original solvent used in the paint - those solvents can be used to remove a lacquer paint long after the paint has dried. Lacquers may appear to "cure" - the surface will appear dry but the paint will still be soft enough to fingerprint for some time after painting - but this is because solvent is trapped under the dry surface and takes a little while to completely evaporate. These two ways of characterizing a paint are often mixed - for example an "acrylic lacquer" is a lacquer type paint with an acrylic (plastic) based vehicle. Since it is a lacquer, the solvent is implied to be a petroleum based "lacquer thinner". As you might expect, "enamels" and "lacquers" have various tradeoffs that affect which you might want to use for a particular application. The big advantage of lacquers is their lack of curing time - air dryed enamels can take weeks to cure to full hardness, while lacquers dry hard enough to handle/sand/polish within 24-48 hours. The vehicle and pigments used in lacquer paints has (at least traditionally) been much more opaque than that in enamels, so when using lacquers you can apply a thinner paint film that doesn't obscure surface detail. Lacquers also tend to dry harder than enamels (even after the enamel cures), which makes them more durable. This means the paint doesn't wear off the parts from handling during final assembly, and is less likely to wear through during polishing. The downside to lacquers is that the solvents used are usually pretty nasty stuff that will attack kit styrene, brush bristles and brain cells with equal glee. The other problem with lacquers is that painting one color over another can result in the top coat re-disolving and mixing with the base coat, resulting in muddy colors. IMPORTANT NOTE: the word "enamel" is sometimes used to refer to any glossy colored finish, regardless of it's chemistry. The most common example of this "mis-naming" is on bottles of nail polish that tout themselves as "nail enamel", when their behavior is usually more like a lacquer. In fact model car builders sometimes use nail polish and treat it just like a lacquer paint - even down to thinning it with lacquer thinner. Until recently, most paints intended for modeling - like the little bottles of Testor's - were traditional enamels, using relatively mild petroleum (or "oil") based solvents with "alkyld" based vehicles [no, I don't know what an "alkyld" is - any paint chemists have a simple explanation?] In addition to the obvious benefits for modeling - relatively safe for plastic and humans, no problems with detail painting on top of a base coat - these paints have drawbacks for the serious modeler: they take a long time to cure, never cure very hard, and go on so thick that they obscure molded in surface detail. But technology marches on... Lacquers have been formulated with more plastic-friendly solvents. Acrylic, and to a lesser extent polyurethane derived "vehicles" make it possible to formulate an "enamel" paint that covers as thin and dries and cures nearly as fast and hard as a lacquer. The ultimate trend in this direction are "aqueous acrylic enamels" - enamel paints with an acrylic vehicle that use *water* as a solvent. ANOTHER NOTE: Modelers often refer to "aqueous acrylics" as simply "acrylics". This is a little dangerous, since there are also "petroleum-based acrylic enamels" and "petroleum-based acrylic lacquers" available, and you don't want to mix up the thinners for these 3 very different types of paints. [Q] So where do "epoxy" paints fit in this picture? [A] (Don Schmitz 11/96) Although I've seen the term "epoxy" used to describe modern enamels that use polyurethane based vehicles, this term usually refers to a two part paint where a seperate hardener is mixed in just prior to application to initiate curing, much like an epoxy glue. The advantage of such a paint is that the curing process takes place throughout the paint uniformly, and so cures much faster, and usually harder than standard enamels. This sort of paint is often used in rugged industrial applications such as on 1:1 aircraft and construction equipment. Dupont Imron was one of the first of these sorts of paint. The down side is that most of these paints produce very toxic vapors (the offending chemical is "isocyanate") that are *not* filtered out by standard respirators. To use these safely, you need a respirator with external fresh air supply, and you should have full skin protection as well. These sorts of paints are really overkill for modeling applications, and I can't recommend that you use them. [Q] What type of paints are best for modeling? [A] (Don Schmitz 11/96) Clearly, this is a subjective question with no right answer. Every paint has good and bad properties, it is up to you to decide which best fits your particular needs. Following are a few observations and opinions: Water-based "aqueous acrylics" are generally considered to be the wave of the future. They combine the best properties of enamels and lacquers while being the least toxic, and can be cleaned up (before they cure) with soap and water. To be honest, these paints aren't quite the equals of the old-gaurd petroleum based paints - they don't dry quite as quickly or as hard - and being relatively new to the market the color selection isn't nearly as wide as the alternatives. But, they keep getting better. If you are just getting into modeling and don't have a huge stock of incompatible petroleum based paints "aqueous acrylics" are probably the way to go - they certainly merit a look-see. Even if you are currently happy with petroleum based paints, as air pollution laws get ever tighter it is possible that some day petroleum based hobby paints will be regulated out of existance - so you might want to think about a gradual switch-over now. "Petroleum-based acrylic enamels" also have a lot going for them. These paints are basically new-tech versions of the traditional enamels Testor's has been selling since forever in those little bottles. Also known as "modified enamels", these paints have an acrylic vehicle that dries much quicker and harder than traditional alkyld enamels, to a near lacquer like hardness in 2-3 days. They may also have a slightly stronger solvent that may attack some kit styrene, but this is easily avoided by using an automotive style primer under the paint. Many of the hardware store spray paints - like Krylon and Red-Devil are this type of paint. One last bonus - a can of Krylon spray paint at the hardware store or local K-Mart usually costs less than $3 for a 12oz can, about the same that Testor's charges for a 3oz can of hobby paint. [Q] What is paint thinner? How do I use it? [Q] Do I need to use those expensive little bottles of thinner made [Q] by the paint companies? [A] (Don Schmitz 11/96) A "thinner" is the generic name for a solvent that is compatible with the solvent in a paint. It is added to paint to reduce the viscosity of the paint. Since it is much easier to add solvent to a thick paint than remove it from a thin one, and since airbrushes require much thinner paint than a bristle brush, most paints are manfactured on the thick side, with the expectation that the user will add thinner to get the appropriate viscosity. Thinners can also be used for cleaning (air)brushes after painting. NOTE: It is important to mention that thinners are usually not exactly the same as the solvent in the paint. One side effect of many enamel paint thinners is to slightly accelerate curing of the paint, much like the hardener in an epoxy paint. This is generally a good thing - unless you've added the thinner to a new, full bottle of paint and then use a small amount. The thinner can cause all of the remaining paint to cure in the bottle, turning into rubbery goo in a few days time. Its best to mix the thinner in a separate container - such as the airbursh paint bottle. An old coffee can lid makes a good pallette for thinning small amounts of paint when brush painting, and a tooth pick is a good way of transfering small amounts of thinner to a puddle of paint. Most paint manufacturers package a thinner that is compatible with their paints in the same small bottles that the paint comes in, usually at the same price. If you're familiar with non-modeling paints, you quickly recognize that the stuff in those little bottles - usually costing several dollars per ounce - looks, smells and acts a lot like the mineral spirits, lacquer thinner and rubbing alcohol that you can buy at the hardware or drug store for a few dollars per *quart*. As you might expect, many modelers use these cheaper products instead of the hobby shop versions. In general, there is no problem with using these products for clean up work - but trouble can arise when actually using them to thin paint. Certain combinations of hardware store thinners and hobby paints that you think should work together can turn out to be incompatible, with the results being the paint turning into rubbery goop in your airbursh, never drying, or yielding a cracked or lumpy surface on your model. There are two approaches you can take to avoid such problems: 1. Use the hardware store products only for cleanup, while using the hobby paint manufacturer's thinner when mixing with their paint. You don't use all that much thinner this way, and you can be reasonably sure the manufacturer has optimized the thinner for the paint. 2. Experiment with various hardware store thinners until you find a combination of products and technique that work well, and then be consistent in using exactly those products and techniques. One suggestion is to avoid "store brand" paint products, as they may come from an assortment of manufacturers and thus vary slightly from batch to batch. The listing of paints at the end of this section includes suggestions for alternative thinners for each particular paint. [Q] What is primer? Do I have to use it on my models? [A] (Don Schmitz 11/96) Primer is a special type of paint meant to prepare a bare surface for subsequent coats of paint. Primers have several special properties: - Primers "stick" to surfaces very well. Typically they have a very strong, very volatile (evaporates fast) solvent that eats through traces of skin oil and such that may be on the surface and mildly etches the plastic so that the primer's vehicle can form a good mechanical bond with the surface (by solidifying inside all of the microscopic pits in the plastic). The primer itself then drys to a microscopically rough surface that allows a good bond to form with the subsequent paint. - Primers are very opaque, providing a consistent base color so that puttied areas and parts molded in different colors don't show through light colored top coats. - Primers typically contain some amount of filler material that helps to fill *minor* sanding scratches and low spots. Primer is not a replacement for puttying and sanding. - Primers provide an easy way to check and correct your surface preparation before putting on paint. Blemishes such as poorly filled seams, low spots and pinholes are often hard to see on bare styrene mottled with putty, but jump right out under a coat of medium grey primer. Primers generally sand well and "spot in" easily, and "lacquer style" putties will stick right to the primer, so you can go back and re-putty and sand, then shoot some more primer over your fixes. - Primer can acts as a barrier between the plastic and some of the more aggressive paint solvents, such as those in "modified" (hardware-store brand) enamels. In general, primers won't protect styrene from real lacquer paints - there are special sorts of primers just for this purpose. The hobby paint makers all produce primers, however many modelers use hardware store brand primers like Krylon and Plasti-cote in spray cans, or automotive primers to shoot through an airbrush. Almost all primers are lacquer based, which means they might attack kit plastic, but usually they dry so quickly (due to the highly volatile solvent) that there isn't time for real damage to occur. As always, you should experiment on scrap parts with new-to-you paint products before using them on a rare model that you've invested 100s of hours in. The only real downside to using primer is that it adds more steps to the painting process that often aren't strictly necessary. In addition to applying the primer, you have to wait for it to thoroughly dry - usually 12 to 24 hours - and for a high gloss surface you should finish wetsand the primer with 600 grit sandpaper, followed by a good soap-and-water wash and air dry, prior to applying the paint. Whether or not to use primer all the time is somewhat of a religous debate. Most petroleum based hobby enamels will stick well enough to clean, bare styrene, and if the parts are a light colored plastic with no putty you won't have any problems with coverage - using a primer in this situation is probably a waste of time. On the other hand, many modelers use primer as part of their painting routine to avoid being surprized by cases when they really needed it. The choice is up to you. [Q] Is it safe to apply one type of paint over another? [A] (Don Schmitz 11/96) The quick answer is "sometimes - always try the combination on scrap before applying to the model". More generally, it is *usually* safe to use one type of paint on top of a paint with a stronger solvent, as long as the base layer is thoroughly dry and cured. Practically, this means you can: - use most any type of enamel paint over a lacquer paint. - use most any type of enamel over the same type of enamel. - use oil-based hobby enamels over lacquers or modified (hardware store) enamels. - use aqueous acrylic enamels over most any other type of paint. This is just a "rule-of-thumb" - you may be able to use other combinations, and these aren't guaranteed to work for every paint variant. [Q] Is it safe to mix different brands of paint to get a particular color? [A] (Don Schmitz 5/97) This will sometimes work if both are similar types of paint (eg. both are petroleum based enamels, or both are lacquers, or both are aqueous acrylics. This is really a bit of a crap-shoot, you really have to experiment to find out what will work. [Q] I put a second coat of the same type of paint on my model and got [Q] a cracked and wrinkled mess - what happened? [A] (Don Schmitz 11/96) Some enamel paints (reportedly Testor's Model Master line in particular) have strict limits on when you can apply a second coat. The problem is that the solvent in the paint can upset the curing process in the base coat. If you recoat quickly - within an hour or so - the curing process hasn't gone very far and the second coat just adds to the first. If you wait a little longer, the base coat has started to cure but is still somewhat soluble in the paint solvent, resulting in the base coat getting soft and possibly losing its bond with the surface. With such paints, you have to wait for the paint to fully cure (as much as a week) before applying the second coat. Some paint labels will give explicit recoating restrictions, and some enamels can be safely recoated at any time, but if you can't find this information your best playing it safe and waiting a few days between coats. [Q] Why doe my "flat" paints come out glossy? [A] (Don Schmitz 11/96) How glossy a coat of paint appears is determined by how rough or smooth (on a microscopic scale) the dry paint surface turns out. Flat paints usually contain a powdered filler, such as talc, that makes for a rough, no-gloss surface. Flats also tend to use a highly volatile solvent so that the paint droplets don't have a chance to flow out before drying. If you don't mix the paint or shake the spray can well, you can leave the talc on the bottom and get a too smooth, semi-gloss finish. Or, if you thin with a slow drying solvent the paint will have a chance to flow out, also giving a semi gloss finish. If spray paiting, your technique can also affect the finish - to get a really flat finish put on lots of thin (fast drying) coats as opposed to heavy "wet" coats. Finally, each manufacturers idea of how flat "flat" should be may differ, so compare paint lines. [Q] Can I get a good high-gloss finish with a brush? [Q] Do I need an airbrush to build good models? [A] (Don Schmitz 11/96) It is possible to get a smooth, high gloss finish using a (bristle) brush. The trick is to use a high quality, fine bristled brush and to get the thinner/paint ratio just right. As in spray painting, you want to put on a wet coat of paint just short of running, so that the brush marks have a chance to flow out before the paint dries. I've done this once in my life - purely by accident - but it was truly amazing to see that glass like finish appear behind the brush. Like all modeling skills, practice and experience should allow you to do this more consistently. Spray cans (aka "spray-bombs" and "rattle cans") are a reasonable alternative to an airbrush. Many modelers use spray cans for applying primer rather than bother with setting up, tearing down and cleaning their airbrush. With the advent of paint polishing kits (see section 7 of this FAQ) you can get a very high gloss finish with a rattle can of hardware store paint. The downside is that spray can paint tends to go on heavy and so can hide surface detail, the color selection is limited, and you can't paint a small area/fine line such as a camo pattern. Still, many modelers make do without an airbrush, relying completely on spray cans and clever masking to achieve amazing results. Once again, practice makes perfect. [Q] What is "Future Floor Wax"? What is it good for? [Q] What do you thin Future with for airbrushing? [A] (Don Schmitz 1/97) Future Floor Wax is a product of the Johnson Wax Co, and can be found in most US grocery and discount department stores (like Wal-mart/Kmart) that sale housewares. The same product is sold under the "Kleer" name in other parts of the world. Price is about $5 US for a bottle that holds at least a quart (a liter for you imperially challenged types). Despite being billed as a "floor wax", Future is really more like a clear aqueous-acrylic paint. It is the right consistency for airbrushing straight out of the bottle, so you don't have to worry about what to thin it with. Future tends to be very forgiving - it doesn't "orange peel", doesn't seem to yellow, and takes well to sanding/polishing so runs aren't fatal. Given the cheapness, modelers have found numerous uses for Future in addition to the obvious one of using it as a clear top-coat. Some of the often mentioned uses are: - To form "lenses" on instruments. After detail painting the needles and numbers, add a heavy drop of Future to the instrument faces. - To cover scratches/dull spots on clear parts - just dip the parts in the Future to get a nice wet coat, let the access run off onto paper towels or such, and wait for it to dry. Common experience seems to be this stuff never yellows. - As a "base" for improvised paints/tints. Folks have suggested adding ink or food coloring to Future to create transparent colors for tinting windows and such. Or mix in some aqueous-acryling paint or pastel dust and use for weathering purposes. A few caveats on using Future: - Some decal setting solutions - Solvaset in particular - seem to eat/etch/cloud Future. If the effect is minor, you can polish it out or fix it with another coat of Future, otherwise be prepared to strip the Future (and your decals) off and try again. - Using Future over Tamiya aqueous-acrylics can have horrible results, unless the Tamiya paint is *really* dry - as in several weeks or more. The problem seems to be that the Tamiya paint can take a long time to dry/cure, and shrinks slightly as it does. The Future dries much faster and doesn't shrink. The result is that Tamiya paint tries to shrink while it is firmly stuck to the Future top coat, which is rock solid. Eventually enough stress builds up to cause the Tamiya paint to crack. This typically happens a few days/weeks after putting down what at first seems to be a perfect Future topcoat! Only solution is to strip it all off and start over. [Q] Is there any way to speed up the drying/curing process? [A] (Don Schmitz 1/97) The time spent waiting for paint to really dry/cure - as long as a month for old-tech alkyld enamels and some of the early formulations of aqueous acrylics - can be a real drag. Many modelers will switch to some other project while a model is in this state, but this time-off can really drain the enthusiasm for a project. It is possible to speed up the dry/cure time of most paints by raising the temperature - even a slight increase in temperature can cut the down time in half. The trouble is doing this without melting the model itself. A reasonably safe temperature is in the 110-120 degree Farenheit range. These are typical of the summer-time temperatures encountered by models in a delivery truck on the way to the store, so they shouldn't hurt the kit plastic. The problem is maintaining a model at this temperature for a period of time: most household ovens have a "low" temeperature of 150 or 200 degrees and have fairly large temperature swings, and if you're married you would need a most understanding spouse to get away with this. The best solution seems to be an el-cheapo "food dehydrator". These can be found at discount stores in the $20 range. They are simply a bucket-like plastic housing with a low-wattage heating element and a number of vent holes at the bottom, and a number of plastic racks that stack on top of the base. The racks are only a about a half inch deep, but its fairly easy to cut the center out of one or a few, or fashion a cardboard spacer so that larger model parts will fit inside. As usual, it will take some experimentation to figure out how long to bake - the guidelines I've seen suggest 24-48 to hours for the most slow drying paints. I toss parts painted with Krylon petroleum-based acrylic enamels in overnight (12-24 hours) and get a *really* hard finish that doesn't wear off with extensive handling. Just be careful when you first take the parts out: while hot the paint is soft and fingerprints easily, although this changes rapidly as the parts cool off. [Q] Why doesn't paint cover the edges of panel lines? [A] (Don Schmitz 5/97) A common problem, especially with colors that don't cover well to start with such as yellows and whites, is poor coverage over sharp edges. The edge causes the surface tension in the wet paint film to drop, and so the paint film thins out over the edge and doesn't cover well. You wouldn't think engraved lines would cause a problem, but the molding process can result in the top edge of the line "puckering" up to a very sharp edge (exagerated ascii art)showing a cross section of a "puckered" panel line): ____/| |\____ |__| The fix is to block-sand over panel lines before painting - and if the paint has a real coverage problem drag an Xacto knife along the edge of the engraved line to smooth the edge slightly. [Q] How do I prepare parts for painting? [A] (Don Schmitz 5/97) A big part of how well painting turns out is based on the preparation of the parts before actually applying the paint. For small parts without large flat areas a good warm-water wash using unscented dish-soap and a thorough rinse under the faucet is all you really need (of course you should file/sand to remove parting lines and such first). This removes any sanding dust, dirt, finger oils, mold-release agents, etc, that might show up in the paint or cause adhesion problems. Resin models are more likely to have mold-release on them (word from the manufacturers is that a modern, well designed and tooled injection mold does not require mold-release) - many resin builders use "Wesley's Bleche White" (whitewall cleaner from the auto section of the department store) to thoroughly clean resin parts prior to painting. Large smooth surfaces - such as plane fuselages or auto bodies - require a bit more work, as irregularities in the paint or underlying surface will be much more visible. Here is the system I use: 1. Fill in any scratches, sink marks, etc with your favorite putty. 2. Sand the puttied areas, and remove any mold lines on the parts, by wet-sanding with 320 and 400 grit wet-or-dry (automotive) sandpaper. If at all possible, block-sand these areas (use a flat block of wood or rubber to back the sandpaper to insure a flat surface). You can block sand most any surface except for concave curved surfaces, but be careful not to flat spot curved surfaces. A small diameter piece of rubber tubing makes a good sanding block for convex curved areas. While you're sanding, lightly block-sand over engraved panel lines, as the molding process often leaves a slight ridge on the top of the engraved line that makes for poor paint coverage. 3. Wet-sand the entire part with 600 grit wet-or-dry sandpaper, again block-sanding if possible. 4. Wash with dish soap, rinse thorougly and allow to air dry over night. If you don't wait at least overnight before proceeding, water will hide somewhere - in a panel line or crevice - and blister subsequent paint. 5. Primer the part with your favorite sandable primer. 6. Carefully inspect the part. Repeat steps 1-5 a few times until the mold lines and surface flaws are really gone. 7. Wet-sand the primer with 600 (or 1000 if you can find it) grit wet-or-dry sandpaper. 8. Wash again with dish soap and allow to air dry over night. You should now have the perfect base for applying the actual paint. [Q] What is a spray booth? Why do I need one? [Q] How can I build a cheap spray booth? [Q] Are bathroom fans/range hoods safe for a spray booth? [A] (Don Schmitz 2/97) A "spray booth" is basically a box with an electric fan in it, vented outside of your house/apartment, that you use to spraypaint parts in. The fan sucks out the paint fumes so the smell doesn't end up in your house and pulls the overspray away so it doesn't land on your model, leaving a rough finish. Being enclosed also helps keep dust from landing on your freshly painted model. You really need a spray booth to safely spray paint indoors, and it can help to improve the quality of your paint jobs. A few companies offer ready-built spray booths, at prices in the $150-300 range (or even higher for big commercial versions). You can find advertisements for these commerical units in the modeling magazines, and well stocked hobby shops can probably order them for you too. [Recommendations for particular make/model would be appreciated - Don] Being modelers (ie. frugal and good with tools), many folks decide to build their own spray booth. There are lots of alternatives for an enclosure - you just need a box big enough to hold a model plus some space to manuever an airbrush or spraycan. Figure roughly 24 to 36 inches wide, 18 to 24 inches tall and 12 to 15 inches front to back as a reasonable size for most modeling jobs. Modelers have used old kitchen cabinets, "Rubbermaid" style plastic storage containers, and even cardboard boxes as the basis for their spray booths. A major source of controversy on RMS is what sort fan to use. The most commonly suggested options are bathroom vent fans, range hoods and the "muffin" fans used to cool electronics equipment. While being readily available and cheap, the problem with this type of fan is that the electric motor is mounted in the air flow (and in this usage, in the flow of air and paint fumes). Everytime these types of fan are suggested, numerous RMS participants will warn of the dangers of sparks from the motors in these fans igniting the solvent fumes and causing an explosion or fire. Actually, most of these fans use motors with a brushless design that do not produce sparks. Despite all of the warnings, no one has ever had any sort of problems with this type of fan - there aren't even friend-of-a-friend style reports of anyone burning down their homes due to this problem, but you have to decide what level of risk you want to take. Safety issues aside, the other problem with this type of fan is that they don't flow enough air to deal with the amount of fumes generated, especially if your are painting with a spray can. I've seen suggestions for using a fans rated at 400 CFM (cubic feet per minute), compared with 50-100 CFM for a typical bathroom vent fan. It may take a little more digging, but you can generally find heavy duty fan-motor combinations at surplus stores at reasonable prices. In addition to moving more air, these types of fans often have the motor mounted outside the air stream for greater safetey. Once you've chosen a fan, you want to mount it in the box and rig some way to mount a pleated-paper furnace filter (a few dollars at a hardware store) in front of the fan to keep paint overspray out of the fan. You'll need to wire the fan somehow, with a switch, and if your safety conscious a separate fuse. If you're not comfortable with basic electrical wiring you should find someone who is - while I don't think you can blow yourself up using a bathroom fan, it is easy enough to electrocute yourself if you manage to connect 120VAC to your body. While you're doing the electrical work, add a small flourescent light fixture inside the box - about $10-20 at the hardware store - so you can see what you're painting. Finally, you need to vent the box to the outside. The bare minimalist approach is simply fit the box in a window opening, much like a window mounted airconditioner - and have the fan vent directly outside. If you don't have a conveniently located and sized window, or you want a neater and more permanent installation, you can use a clothes dryer vent kit (about $10 at the hardware store) to vent your spray booth out a nearby window or through an outside wall (FYI - making a 4 inch diameter hole through a concrete block basement wall is a *lot* of work - plan on spending 4+ hours with a hammer and cold chisel). If you're safety conscious, you'll want to use sheet steel vent pipe instead of the plastic hose that comes with the vent kit to connect the fan to the exterior vent.
rec.models.scale FAQ, part 18

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