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Preparing Your Aircraft For Painting Part 1
Final Preparations and Painting Tips
By Tony Bingelis (originally published in EAA Sport Aviation, July 1985)
Ever spray-paint an airplane? An auto? Didn't take too long, did it? But, how about the pre-paint surface preparation? That took weeks longer than you expected, didn't it?
It doesn't take a new builder long to conclude that the pre-paint preparations can and do take many hours . . . about 100 hours on the average. (How many weeks would that be with your work schedule?) One bright note . . . the actual painting (spraying) will consume less than 5% of those hours.
Once you start there will appear to be no end to your final surface preparation as you discover unfinished areas, and continue to find imperfection after imperfection. Also time consuming, but equally essential in the preparation process, is the acquisition of necessary equipment and materials, masking and re-masking the airplane, and the taking of steps to provide protection for anything and everything within the paint spray area that you do not want painted by the overspray. All this will take many hours of preparation. Just how much work will be required of you to bring your homebuilt project up to the moment for spraying is difficult to determine beforehand. Any estimate of hours will vary with the type of construction and the way the airplane was built.
An all-metal project, for example, should require the least amount of preparation, probably as few as 75 hours. On the other hand, an older all-metal airplane, previously painted, could consume well over 100 hours of preparation work because the old paint would have to be stripped off first.
A "built-from-scratch" composite airplane will consume as many as 200 hours of preparation effort (all those "quick-build" claims notwithstanding). The total number of hours required here for surface preparation is greatly influenced by the quality of workmanship and how well the builder controlled shop rash, surface contamination and construction tolerances.
A composite project assembled primarily from pre-molded components naturally will require far fewer hours of surface preparation because most of its structure is already free of blemishes and is mirror smooth when received.
A wood and fabric or a tube and fabric aircraft project will also take approximately 100 hours of preparation. However, if you plan to apply 20 to 30 hand rubbed coats of finish, scrap that estimate!
All in all, 100 hours spent in preparation seems to be a fairly valid planning estimate for the first time builder regardless of the type of structure he has to paint. Try to keep in mind the fact that the key to a successful paint job is in proper surface preparation.
Preliminary Inspection and Cleaning
Get off to a good start by inspecting every square inch of the airplane to locate and correct those minor imperfections that always seem to escape early detection. Walk slowly around the airplane, all the while running your hand over all the surfaces and especially over the leading edges, wing tips, and control surfaces. Your sense of touch will help you find problem areas that your eyes might otherwise miss. Feel for dents, nicks and imperfections of any nature. You won't catch all of the imperfections but you will find a lot of them . . . many more than you might have expected.
Try to correct any damaged areas located . . . immediately. If you don't, you may overlook them later, or forget where they were - never to be found again until some cad spots them at a fly-in.
This is your last chance to true up wavy surfaces and to fill unwanted depressions in composite surfaces. These will usually require a dry micro filling (a mix of microballoons – Q-cells - and epoxy). Most composite designs have this "fix" detailed in their instructions. Don't use anything like Bondo to fill large areas; it is far too heavy.
Minor scratches and dents in metal can be filled later with one of the spot putties made for this purpose.
During the long period of construction your hands will have pawed all over everything. Hands and fingers leave oily prints and smudges. In addition, your project may have acquired a bit of shop rash in the form of nicks and gouges. In some surface areas, hardened droplets of glue or paint may have affixed themselves where you overlooked wiping them off. You may even have forgotten to trim and file some parts and edges nice and smooth. At any rate, now is your last chance to find these pesky imperfections and to correct them.
Hopefully, you have deferred all greasing and oiling of parts and mechanisms until after you have painted the aircraft. Paint will not adhere properly to any oil or wax contaminated surface. Oil stains are very difficult to remove from wood, composite or fabric surfaces. However, if you have this problem, clean the contaminated areas by wiping them with MEK, Acetone or Prep-Sol (lacquer thinner will not dissolve waxy film, merely spreads the stuff around).
Be sure all lines and markings, particularly felt ink marks, are removed from the surfaces. If they are not you may be sorry later when they bleed through your paint finish . . . I hope you did not use a red marker or red pencil - that color is the worst about bleeding through most any kind of top coat.
If possible blow (air hose) and vacuum the entire airplane both inside and out. (While you're at it, you might as well salvage all those long lost nuts, bolts, washers, screws and tools you retrieve from the aircraft's interior and save them for your next project.)
Check all structural edges and see that they are slightly rounded. Use a smooth cut file and/or sanding block to round sharp corners and to smooth and radius every exposed sharp edge. A slightly rounded edge provides a better grip for the paint and improves its resistance to chipping and cracking.
Wherever possible, remove or delay installing small fittings and parts until after you have the airplane painted. It is very difficult to keep fittings looking good.
Continue your inspection by feel and touch as well as by looking until you are satisfied that the airplane surfaces are in good condition and ready for the final surface preparation.
The Final Surface Sanding
Hand sanding is the final surface preparation for most any kind of surface that is to be painted. This final sanding helps clean and smooth old surfaces but, more importantly, it is used to freshen any surface and provides the necessary "tooth" for better adhesion of the primer (or top coat).
Sanding is equally effective for composite, wood, fabric or metal surfaces. (I understand that even one of the major aircraft manufacturers considers it beneficial to have all their aluminum surfaces lightly wet sanded with number 400 wet/dry paper before undertaking any other preparatory treatment.)
Do most of your heavy hand sanding with the help of a non-rigid sanding block (rubber or foam) in order to obtain a true surface free of waviness and bumps. See Figure 1 for some suggestions.
For light hand sanding (wet or dry), fold and tear a sheet of wet/dry paper into four pieces and fold them as shown in Figure 1. Be careful when using hand-held paper as you may develop an unevenly finished surface because your fingers will tend to exert localized pressures. Hand-held paper also tends to ride over hard spots (small bumps of dried glue or paint) and leave the spots virtually intact. To remove a small localized hard spot or lump, use a smooth file or a hard sanding block to reduce and level the imperfection before continuing your hand sanding. Do not sand into composite structure to remove a large raised area. Make the correction in an approved manner.
If a composite surface is in poor condition, begin with number 180 or 200 wet/dry paper and use one of the larger non-rigid sanding blocks to improve the overall surface. I think it is preferable to sand fiberglass surfaces dry. Next switch to number 320 paper for the final finishing.
Try to do all your sanding with straight strokes, either horizontal or vertical, and resist the temptation to sand with circular movements. This is particularly important during your final sanding because a circular sanding pattern leaves swirls and sometimes sandpaper scratches that may later be visible through the final top coat.
Check the progress of your sanding frequently by sense of touch and try not to remove any more surface material than absolutely necessary.
Some builders prefer to use a hand-held "jitterbug" (sander) to speed up the process. This is quite O. K. as it results in a very nice finish with the minimum of work . . . just don't overdo it by cutting into structural material.
When you have completed the sanding, remove the sanding dust by blowing it away with compressed air. Blow out all the crevices and corners too. Don't forget that the blown dust will become airborne and eventually some of it will again settle on the surfaces.
The air cleaning could be followed by "washing" sanded surfaces with a solvent type cleaner as may be recommended for the particular brand of paint and primer you will be using. Do not wash fiberglass surfaces with water, especially if you intend to paint immediately. You will never be sure that you have wiped and blown away all the water droplets from the corners and crevices. Moisture and paint don't mix, and if both happen to be present when you are spraying, first class problems are in the making.
A metal aircraft with aluminum skins in good condition could benefit from a good final scouring with SCOTCH BRITE Scouring Pads in lieu of sanding with wet/dry paper. The results are excellent and the residue is much easier to clean off than sandpaper dust.
These SCOTCH BRITE Scouring Pads (grocery stores have 'em) are non-metallic and are safe to use. Do not, however, use steel wool on aluminum as any frangible wire particles remaining in the aluminum surface will only invite corrosion. During the sanding process you might want to remove small parts and fittings rather than paint around them. These parts should not be reinstalled until after the aircraft has been painted.
If some of your landing gear parts and other previously painted fittings have become chipped and raunchy, you might consider wet sanding them lightly with number 360 or 400 paper and paint them along with the rest of the airplane.
Equipment and Supplies You Will Need
Here is an advance list of the more important equipment and materials you will need for preparing and painting your airplane. Most items may be found at the average paint store or automotive paint center:
1. Compressor (capable of putting out 80 psi or more and keeping up with your gun).
2. Respirator or Face Mask - suitable for the type of spraying you will be doing.
3. Clean rags and paper towels.
4. Masking Paper- it comes in rolls and in different widths (12" is handy).
5. Masking Tape or Electricians Plastic Tape - for masking.
6. Spray Gun - a good one that shoots a nice circular pattern.
7. Tack Rags-they come in plastic packages - paint stores.
8. Large Electric Floor Fan(s) - use as exhaust fans.
9. Wet/Dry Sandpaper (numbers 180, 320, 360 and 400 grits).
10. Fire Extinguisher - also impose a no smoking rule.
11. A pair of old shoes-you will ruin whatever shoes you are wearing.
12. Paint Filters - paper cone type.
13. Coffee Cans with Plastic Lids -start saving them now to use with solvents for clean-up.
14. Plastic sheeting - to cover everything you don't want painted, and for making a paint chamber.
15. Paint Stirring Gadget - for your electric drill.
16. Lacquer Thinner or Solvent - for cleaning gun, etc. (at least 2 gallons).
17. Paint/Primer/Solvent - compatible to brand and paint type selected.
18. Tweezers - for rescuing bugs caught wading in your paint and for debris removal.
The above listed items are the biggies. If you need or want anything else, add it to your list and you won't have any work stoppage problems due to a lack of planning and preparation.
More next month.