If you are a beginner looking for advice on how to tackle a good old-fashioned dope finish on your model, this article from the March-April edition of American Modeler is about as fundamental as it get. The author recommends methods for operating and cleaning a spray gun, how to properly prepare a model for painting, taping off trim lines, and even achieving that high gloss finish that so many contest grade models have. If you are looking for pictures, then this article is not for you - it's all business. However, if a picture is worth a thousand words, then the inverse must be true that a thousand words is worth a picture. That being so, this article contains three pictures.
Scalecraft & Finish
By Paul Plecan
You want your next scale job to be slicker, smoother, and shinier, right? It takes patience and requires effort.
In this review we will cover Ukie, R/C and F/F in that order. Since control-line weight is not an overwhelming consideration, we can duplicate the "average" finish seen at most contests. Most of us may be familiar with the amateur approach - too many coats of dope too hastily applied, insufficient drying time and improper sanding between. Yes, there is much more of an art to "sandpapering" than meets the eye. It quite often is the hardest skill to master.
For the usual glow-plug equipped model, we need a fuel-proof type undercoating and pigmented dope. Naturally, there are favorites, but this is a personal thing. What works best for you can only be determined by Y-O-U.
Some brands can be mixed, but formulas frequently vary and mixing of two brands can result in a useless goo. So test first before trying unknown combos on a model. The beginner should go the whole route with a single brand. Only deviation we make from the one-brand rule is to use inexpensive thinner for cleaning hands after working with dope or lacquer. While cheap thinner will do for this do not use it with the undercoating or pigmented dope.
To apply the dope to the model, we need brushes. Camel-hair, ox-hair, and sable brushes are long-time favorites - you will get the quality you pay for. Cheap brushes shed bristles and soon come apart entirely. A wider brush will speed up work; 1/2" wide brushes are the minimum to consider, 3/4" and 1" widths are popular. At least four brushes are needed: one for undercoating, the others for light, medium, and dark work (you discover that a brush, once used with black, can not do much of a white dope job). Between two and three dollars is needed for the four brushes, so take care of them.
Never, never allow dope to dry on a brush. Any time you must lay it down, dip it in thinner and lightly squeeze out the dope. A handy way to clean brushes is to have three small (approx. 1-oz) jars lined up, 2/3 full of thinner. Dip the brush in #1, squeeze dry with clean rag, repeat dip-dry sequence with #2 and #3. Keep jars lightly covered, replenish thinner when dirty. Tighten caps when not in use to keep thinner from evaporating. To get quality brushes you may have to go to an art supply store.
An alternate method for applying dope is with a spray gun. A unit rated at a minimum of 3 cubic feet of air per minute is required. Forty or fifty dollars for a new outfit.
Happily, there is an economical alternative in the spraying department. You can get spray cans that are easy to use, or attachments that have separate air pressure cans and dope jars. The latter allow custom-mixing of colors.
For two to three dollars, you get a head assembly, a 4-oz jar, and a 15-oz pressure unit (extra pressure units are between one and two dollars). Each pressure unit can spread almost a quart of dope. It will pay you to have separate units for light and dark work, maybe a third unit for intermediate shades. The reason? It takes a fair bit of thinner through the sprayer to clear out the previous color. Two or three "rinsings" to go from a black to white for instance. There are instructions on the label - read and follow them! When liquid in any integral spray can unit is depleted, always release remaining gas pressure.
Another item on the shopping list is sandpaper. "Bargain basement" paper will not do. At least 6 sheets each of 260 grit and 320 grit waterproof sandpaper. You may find grit designation will vary - 280 and 400 grit is just as good as grades listed above. If sheet size is about 9" x 11", it will be much handier if these are cut into quarters or sixths.
Let's assume your model is fully assembled, all fabric or tissue surfaces having been water-sprayed and dried to tighten up covering. Before we start with the undercoating, clear dope all fabric or tissue areas. Undercoating directly on fabric would overly increase weight.
You apply undercoating with a brush or smooth it out with your fingertips, making it sink into the pores of the balsa. This is our main objective - to fill in the pores.
Allow the first coat 30 minutes to dry, then very lightly skimming the surface, sandpaper away the major imperfections. This first sanding can be done with plain sandpaper of garnet type, no rougher than 220 grit or 6/0. Your thumb and forefinger keep the paper from slipping away, the pressure on the work being exerted by the four finger tips. Lightly, now. Best motion pattern is circular or elliptical. This eliminates the sudden stops of back and forth straight motion, where the sandpaper digs in at the ends of the stroke. Develop an even pressure and never linger in one spot - idea is to remove an equal amount of undercoating all over the model.
In case anyone is confused, the term "undercoat" here means any of the items known as primer surfacer, primer sealer, sanding sealer or balsa fillercoat. A second coat of undercoating applied is allowed 40 minutes to dry before sanding. Use the 260/280 grit waterproof paper from now on - whether you dip it in water or use it dry is not too important. Damp or wet usage is preferred. With the application of the third coat of undercoating, it is a good rule to allow even more drying time ... 50 minutes or more is a necessity. From this point on, use the paper thoroughly wet. A dish partly filled with water should be near at hand. Dip the paper into it as soon as you feel it "drag" when sanding. You, should find the balsa-dust and talc floating away from the paper each time you dip it in the water. If it tends to clog up and lose its "bite," then the finish has not been allowed sufficient time to dry. Butyrate type dope seems to take longer to dry thoroughly and tends to clog the sandpaper quicker, but remains more flexible, even over a long period of time, resulting in less "crazing" or splitting. This probably is due to the use of more plasticizer (castor oil, etc.) in the solution.
The sanding of the third coat should continue until you can see that most of the surface coating has been removed and you are sanding the model again. But there will be a great difference now - if you hold the model near a strong light and look lengthwise at any area, its surfaces should be quite slick. There is a chance that the pores of the wood can still be detected. If a top-grade finish is desired, another coat is called for (plus sanding).
When "three or four" coats of undercoating are specified for a particular airplane, we must remember the biggest "variable" of all is the model-builder. One reads the can labels, shakes and stirs the solution well. Another just zips along, paying slight attention to article, label, or well-meaning friends. This is the guy who needs the extra coat. Maybe more, if he applies it with a skimpy type technique.
By this time you should be some sort of judge on drying times, sandpaper pressure, number of coats. And your handiwork should reflect more truth when the word "workmanship" is bandied about. Use of waterproof sandpaper should not mean that the model stays wet continually. Once sanded, wipe dry with a cloth or crumpled tissue. And even though dope tends to sink in (undercoating, too) when the first few coats are applied, you can sand too deep. If you do, this may let a bit of water penetrate under the doped surface. You'll know when it happens, as just a little water in the balsa wood will cause a blister or puffing up to occur right where the water seeps through. Once this happens, there is almost nothing one can do to correct it in a hurry. A series of punctures with a very sharp X-actro blade or pin in the vicinity of the blister will aid in letting the moisture escape, as will leaving the model in any well-heated dry area. About a foot away from a furnace, or even farther away if placed near a stove. Check the damage within 20 minutes, as you may place the model too close and the finish may blister from heat. Safer to let it dry overnight, not too close to any very hot surface.
You should have a model now that looks great even without color. But the pigments are what we are after in any paint job. Before you proceed with the color dope here's a tip for the "all-out" slick finish fans. To be doubly sure of smooth surfaces, a coat of silver dope at this point will reveal all the dimples, low points, slight pore effect or other blemishes. Nothing else will more critically highlight the slightest imperfections ... unless you want to use a microscope!
On with the colors. Always apply colored dope in smooth strokes of the brush, flowing the liquid on with a minimum of "stroking." This minimizes streaking and gives more even coverage. Keep a well-thought-out program in mind; start with nose, left side, then right side of fuselage. Top left wing panel, then bottom. Repeat for right wing. Repeat wing sequence for horizontal tail surfaces. Left, then right sides of rudder; lay aside to dry. Somewhere along the way now you will realize that each succeeding coat of dope traps all the rest under it. If any previous phase was rushed through, the dope remains "wet" in actual truth even overnight. It is for this reason that overnight drying is desired. The sanding goes that much easier, and less clogging takes place too. You can almost pose as an expert now.
Certain colors have more "body" and hiding power than others. Well-mixed black dope will cover most any previous color in two coats. White takes more than six coats to effectively cover black. So it depends on the particular color scheme as to how many coats are required. If you choose a model with a light-colored paint job, you will find that it takes four coats of light yellow or white to achieve a solid body of color, without faint streaks or blotches. Dark stripes or other trim should effectively cover the light areas with two coats - three at most. If you aren't getting effective coverage, you've let the dope settle too long after stirring or shaking. You'll hardly ever need to change brands, as a poor paint job reflects mostly the model-builder's lack of "know-how" more than the quality of the dope used. There is a limit as to how much you can learn by the "book" - you just have to get your hands dirty.
Which brings us to a minor matter - clothing. Cultivate the habit of wearing your old tattered shirts and slacks when painting. It only takes a drop or two to mess up some of your favorite threads. And getting the pigment entirely out is only a matter of opinion (you'll wind up with some sort of spot in any event). We don't usually notice the spot until the dope has dried to some extent - it's too late then. Shoes can be a problem too, but if you keep them shined, the wax generally keeps the dope from actually getting into the leather. Note that we said "generally."
All light colors go on first. With white, six coats are needed. Most other light colors. four coats. Dark covering coats, three. In extreme cases; you could add two additional coats to each of above.
The use of rubbing compounds is recommended on the "museum piece," U-control stunt or scale or an R/C model that you have invested in heavily, money-wise. Along with the rubbing compound, you'll want to use gloss wax. Read the instructions on the can and use plenty of elbow-grease.
After flying, the sooner you clean your model, and the less time you leave spilled fuel on your model, the better. Pactra's Plane Kleener or rubbing alcohol will cut into the exhaust sludge that streaks the engine cowl and the bottom of the fuselage. Store models in an unheated area of the house, as the average "indoor" portion of most houses is too dry, causing the finish to "craze" in short irregular cracks that generally cover the entire model. With the passage of time, cracks sometimes enlarge so the darker areas split enough for the lighter coats to be seen.
This condition can be prevented on your next painting job by adding a drop of castor oil to each ounce of dope that is used. Castor oil slows down the drying of the dope, calling for a longer wait before sanding. With even an overnight drying period. you will find the sandpaper loading up more more quickly (so you will need about double the usual requirements).
The model-builder who uses a spray gun should follow manufacturer's instructions - and always keep the gun in motion. Maintain an even rate of rotary motion for even coverage, working at a constant distance from the painted surface. You soon learn the vital importance of snug masking and air-tight "cocooning" of lighter areas when working with dark colors. The merest hole will allow some of the darker mist to penetrate to mess up the job.
Unless you have a large work area that is both heated and well ventilated, forget spraying. A warm area (65 to 70°) is needed to get proper atomization and ventilation (through-wall fan duct) is needed to carry off fumes. Without proper ventilation, an average spray session will leave droplets all over you and everything in the spray room. An open door to the rest of the house will mean a dope-mist on drapes and furniture. Always have all the preparatory work finished before loading your spray gun. You don't want the pigments to settle before you get into action. By loading and being ready to spray, you get those pigment particles onto the model, where they belong.
For R/C .models, the same general instructions apply. Do not overload such planes with too much pigmented dope. You already have enough payload in the form of R/C equipment. And on the larger models, weight builds up very rapidly. A model doubled in size has four times the area to paint ... tripling the size multiplies the surface area nine times. Get the dope onto the model right after the pigments have been agitated into suspension; allowing them to settle calls for extra coats that otherwise would not be needed.
For free-flight scale, the old style colored tissue model with tissue trim or numbers cut from contrasting colored tissue and doped on appears dead as a dodo. A full paint job is the order of the day, with flying performance suffering appreciably. Lucky for us, the engine makers have not been sleeping; we can still haul the deluxe paint jobs around effectively. Due to the greater shock hazards inherent in F/F, it is recommended that you plasticize the dope even more than for Ukie jobs. Two drops of castor oil per ounce of pigmented dope for tissue-covered models. So on the flights where the model whacks a pole, tree-trunk or brick wall there is less chance of splits in the tissue covering. Silk, being more resistant, will not need heavily plasticized dope ... no more than 1 drop per ounce.
Weight being an important factor in F/F scale, you want the maximum pigment coverage with the minimum number of coats. Since it takes so many coats of white or other very light shades to "cover," stay away if possible from designs that call for light paint schemes. Good pigment coverage is had with red, blue, olive drab and other middling-to-dark shades. Silver covers very well in just one or two coats. But it accentuates every wrinkle and scratch in the surface of the model.
How to pull masking tape away from a paint job without tearing the covering or "lifting" the edge of the paint job? When using a brush to apply pigmented dope along the edge of masking tape, try to avoid painting the tape. Paint only to the edge if you can. Naturally, you will "miss" some of the time. Second, don't overdo the number-of-coats bit. With two dozen coats, you've practically bonded the tape to the model. On many coats, peel off early, when dope is comparatively pliable. On few coats, peel late, dope can be quite dry. Pull tape off with it doubled back running along on itself 180 degrees. This gives the maximum shearing action where the tape is parting company with the model. And pull in a slightly side-wise manner, away from the doped area. The worst you can do is pull at right angles to the work surface - then you are trying to peel the tissue off too, to some extent.
To mask off long sweeping curves, a template is needed, to be sure that the curve is the same on both the left and right sides of the model. On intricate or small lettering or trim jobs a steady hand is called for.
Where lettering or trim exceeds 1/4 to 1/2", a patch of masking tape is laid on glass, the design then cut into the masking tape with a frisket knife with point razor-sharp. Granted, it is a major problem just lifting the masking tape "stencil," aside from properly positioning it on the model. But the results are worth the effort. It is the only way a design can be painted on a concave or convex area. A decal will not lay flat in such an application. On flat surfaces, there is no problem. Fuel proof decal sheets are available in almost any color and can be cut to almost any required shape.
Work with care. A poor construction job can not be hidden under the best paint job. When the model is built well, when high quality finishing ingredients are used, the result is a delight to the eye.
Posted December 30, 2012