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Preparing Your Aircraft For Painting Part 2

Final Preparations and Painting Tips

By Tony Bingelis (originally published in EAA Sport Aviation, September 1985)

You worked hard and long on the pre-paint preparations outlined last month and you have reached the point where you feel that any further sanding, filling and inspecting of your project would be counterproductive, a waste of time . . . and, by golly, you are ready to prime this baby right now. But wait!

Do you know what kind of primer to get? You won't know that, will you, until you decide what type of paint you are going to use. Have you made that decision yet?

The primer and the paint you select must be chemically compatible with each other. Furthermore, the thinners and the solvents you will be using must also be compatible with the primer and paint. This usually means that all of them should come from the same manufacturer.

Using a primer from one manufacturer and the solvents and thinners from another is risking the quality of your paint job. It is very important that you do not mix brands. Even though the thinners and solvents seem compatible, they may not be. They may be chemically different, and cure or dry at different rates. If so, this may induce surface stresses in your finish coat and lead to a botched up inferior appearance. Adhesion, too, may be poor and sometime later your paint may begin to lift and peel. That would be enough to make a grown guy cry. But there are other pitfalls, too.

You can still cause yourself a heap of trouble even when you don't mix brands if you don't familiarize yourself with the manufacturer's recommendations for the paint type selected. You cannot, for example, ordinarily put a lacquer topcoat over an enamel primer even when it is a same brand product. The lacquer will cause the enamel undercoat to curdle and lift under the more volatile lacquer solvents. The results are predictable -- one big mess.

Stick with a single good brand for both the surface preparations and for the finish coating. Follow the manufacturer's recommendations for the primer and for the proper solvent to use. (Solvents and thinners are selected to suit temperatures expected to prevail during the spraying sessions.)

Just in case you have no idea what brand of paint to select, you might like to know that many amateur aircraft builders prefer the paint products identified informally as COOPER, DITZLER, DUPONT, RANDOLPH and STITS.

Leading the pack in popularity and excellence, locally at any rate, are the DUPONT and the STITS brands . . . DUPONT, perhaps because of its large variety of paint products and the EAA Chapter 187 discount we enjoy with those folks; and STITS because of its fine reputation and general availability.

Like most builders, you probably will have heard all sorts of claims of excellence for some particular "best" paint to use and, on that basis, you think you want to use it, too. Let's think about it. The most popular type of paint (not brand, TYPE) may not be suited for the kind of aircraft surface you have to paint.

Broadly speaking, we have five different paint types or topcoat finishes from which to select. There are, of course, other types of finishes but the following five are the mainstay of aviation and automotive artisans:

Dope Finishes Enamels
Epoxy Finishes Lacquers
Polyurethane Enamels

Each type of finish is quite different from the others in durability, rate of drying, resistance to corrosion and in the types of surfaces for which it is best suited. Let's look at each of them briefly.

Butyrate Dope Finishes
Everyone knows that dope is used for fabric surfaces. What everyone doesn't know, however, is that dope cannot be used over metal surfaces because it will surely peel off. It is not suited for use on slick composite surfaces for the same reason.

Since all fabric covered airplanes have quite a few metal parts and surfaces, in addition to their fabric covered components, you will have to switch to some other type of paint for the metal work. This may make it difficult for you to obtain a good color match to go along with the doped parts. Even when the color match is just about perfect, the difference between the sheen of a doped fabric surface and the gloss of a painted metal component is often pronounced. Having to switch from dope to paint and vice versa is, to say the least, troublesome.

One nice thing about using dope is that it can be easily touched up, patched and blended without repainting the entire unit . . . something you can't do as successfully with enamel.

Because dope dries fast it can be sprayed out-of-doors without the benefit of a paint booth . . . provided a little extra care is exercised in selecting the best conditions for the paint job (early morning hours, no wind, low humidity and warm temperature). Dope is humidity-sensitive and you should not spray it when the humidity is over 65% (best temperatures are between 65 and 80 degrees F).

Dope doesn't cover as well as many other types of paint and, as a result, you have to be prepared to apply numerous coats of dope to equal the coverage obtained with only a couple of coats of, say, acrylic enamel on other types of surfaces.

In short, doped surfaces can be work intensive. They require, first of all, a brush coat of Nitrate dope followed by two or more coats of Butyrate dope, and then a couple more coats of pigmented aluminum dope. All this is followed by three or more color coats of Butyrate. Of course, a lot of different coats spells a lot of work, a lot of wet sanding in between and a lot of rubbing. The STITS process using Polybrush and Polyspray is somewhat different and a lot simpler to use. Nevertheless, either product is still the best paint type to use on fabric covered surfaces, be they grade A fabric, linen or one of the Polyfiber fabrics. The finish will remain quite flexible throughout the years of service and is less likely, by far, to crack from embrittlement than when finished with any other type of paint currently available.

Enamel is a paint with varnish in it. This explains the instant shine that is characteristic of an enameled surface. The paint dries fairly fast (dust free in 45 minutes, about half that time for acrylic enamels) but is slower drying than a lacquer or Butyrate dope coating and, consequently, there is more time for dust and bugs to take refuge in the tacky surface. Painting with enamel should, therefore, be done indoors, preferably in a paint booth. It is, nevertheless, a very good paint type for use over most any kind of hard surface. It is very durable and holds up well under most conditions. The acrylic enamels (Ditzler's DELSTAR/DuPont's CENTARI) are very popular and economical to use.

Enamels are less brittle than lacquers. Nevertheless, they are not very well suited for fabric covered surfaces. One exception might be white DUPONT DULUX enamel, provided it is applied sparingly to the fabric surfaces. The dark colors tend to be more brittle than the whites.

One of the advantages of using acrylic enamel is that the entire airplane can be painted, including the metal parts, without a lot of extra masking preparation.

As a rule, you should never attempt to paint over an enamel finish with lacquer unless you first apply the appropriate sealant recommended for the brand of paint in question. A fresh coat of lacquer will generally cause an underlying enamel surface to curdle and pucker. Ordinarily, there is never any difficulty in applying an enamel topcoat over a lacquered surface.

Epoxy Finishes
Epoxy primers are great but the use of epoxy as a topcoat finish may not be quite as good. Epoxy does have superior adhesion qualities and is quite durable and long lasting. However, it does have the reputation of fairing poorly in the sun, and is prone to oxidizing . . . chalking, if you will.

Epoxy should not, ordinarily, be used over zinc chromate as there is sometimes an adhesion problem in doing so. Use an epoxy primer under epoxy topcoats.

Catalyzed epoxy is rather slow drying (2 to 6 hours) so you may wish to speed up the drying process. You can do this by mixing the epoxy with its catalyst and setting it aside for one-half of its pot life. Then stir it thoroughly and spray it on. The epoxy coating will then cure in half the usual time. This is still slow by dope, lacquer and acrylic enamel parameters.

Being a slow drying paint, it is very difficult to get a dust-free and bug-free finish unless you paint indoors in a well ventilated paint booth.

Lacquer is somewhat like dope in that it dries fast. So fast, in fact, that the bugs in the area have to be on standby alert in order to zoom in quickly if they hope to land on a wet surface. The slower and aerodynamically inept bugs are more likely to hit an already dry-to-the-touch surface, skid off and break their fool necks. What a boon for the gent who has to paint his aircraft outdoors.

Unlike dope, lacquer becomes very hard and brittle when completely cured and will crack and chip "at a drop of a hat" (hard hat), particularly on sharp edges and corners. Obviously it is just about the poorest type of paint you could select for use on flexible or fabric surfaces.

On the other hand, lacquer is more versatile than dope and it can be used on most any type of solid surface (aluminum, steel, wood, composite, etc.). It is a very economical type of finish and is easy to apply. Like dope, you can touch it up and spot areas without repainting the whole component.

Lacquer's quick-dry characteristic can turn into a disadvantage if you aren't careful when spraying. Your paint application must go on wet or it won't adhere very well and the resulting surface will feel rough and fuzzy under your hand. If this happens, sand off the roughness and try again.

Acrylic lacquers are akin to dope in another respect. They also have a low solids content and they cover rather poorly. Actually, five or six coats of lacquer must be applied. Fortunately, all of these can be applied in a single spraying session if you allow about 5 minutes between coats to permit the paint to "flash oft". Actually, there is virtually no wait between coats for larger components because it will take you that long to spray from one end to the other.

There is one more thing you should know about lacquer. A lacquer finish has to be compounded, or rubbed, to improve its gloss. This, however, should be delayed until the next day after an overnight dry.

Polyurethane Enamels (Imron, Durethane, Etc.)
That "wet look" so typical of polyurethane enamel finishes has really captured the fancy of the aviation community, amateur builders included.

These polyurethane paints adhere well to most surfaces. They provide a hard surface finish that is outstanding in gloss retention, resistance to chemicals, fuels and oils, and is flexible enough to exhibit superior chip resistance . . . and, oh yes, this super paint is expensive to use.

This type of paint requires the addition of an activator. Once mixed, however, application is as simple as spraying enamel or lacquer.

Being a rather slow drying paint (about 45 minutes to a dust free cure and 4 to 6 hours for a tape free dry) it is best that your painting be done indoors in some sort of well ventilated booth or enclosure.

Polyurethane enamels are being used on just about every type of aircraft surface. As for fabric covered surfaces, although DuPont (maker of Imron) may not be ready to claim that Imron is suitable for use on fabric surfaces, that doesn't bother some homebuilders. Amateur builders are using Imron on fabric covered airplanes with eye-catching results. Only time will tell how well the finish holds up on those flexible surfaces. So far, the performance of the paint is outstanding.

Perhaps the greatest deterrent to the use of Imron, or any of the polyurethane enamels, is the reputation it has regarding the physical hazard posed by its fumes and spray mists. These fumes and mists contain isocyanates which are highly toxic to the careless or poorly informed user who fails to heed the warning labels on the paint cans . . . and who fails to protect himself adequately during the spraying operation.

The use of polyurethane enamels is especially inadvisable if the builder has a respiratory problem of any sort, or if he has to paint in a confined area with poor ventilation.

Something most of us don't realize is that these paints are intended for professional use only. Professional painters are expected to have the necessary facilities and equipment and are, supposedly, well versed in the recommended safety precautions.

Warning Label Is Not To Be Ignored
There is a very good reason for the Warning Labels manufacturers put on their products. The government requires it and the manufacturer, knowing the chemistry, peculiarities and limitations of his own products, is only too happy to comply (product liability, you know). Paint fumes and spray mists are toxic to varying degrees. Read the entire label and be sure you understand what the manufacturer is telling you about the toxicity of his paints, primers and solvents. Avoid creating a hazardous condition for yourself and your home environment. Ignore the warnings and you may be incurring a greater risk than you may realize at the time.

Paint Spray Respirators
Obtain a good paint spray respirator and use it conscientiously. Actually, you should wear it while mixing your paint and even while cleaning the spray gun with solvents. Unfortunately, most of us do not.

A simple dust mask or surgical mask is not a suitable respirator for spraying paint. It is O.K. for use as a sanding mask and little more.

Sears has an excellent paint spray respirator (918552), with replaceable chemical cartridges, that will afford protection against enamels, lacquers and dope finishes. It WILL NOT protect you against the isocyanates contained in polyurethane enamels even though the respirator looks formidable enough to protect you from everything short of an all out gas attack.

Just because a paint spray filter has chemical cartridges and has an impressive MSHA/NIOSH (U. S. Governmental agencies) approval for it doesn't mean it can be used when spraying any type of paint. Be sure that your paint spray filter includes an approval for the type paint you will be using.

How-To-Do-It Type of Information
All of the paint manufacturers previously mentioned put out useful manuals, brochures and guidance for the use of their paint systems and finishes. This is the kind of authoritative guidance you should follow when using their products, and nobody else's. They best know how their particular formulations of primers, paints and solvents should be used. They want your paint job to turn out perfect . . . they, too, have a reputation to protect.

The nearest local distributor for these products can probably provide you with whatever information you need and furnish excellent "how-to-do" brochures as well. Some builders, however, might find it easier to write to the manufacturers listed below:

More next month.

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