Painting Your Homebuilt-Part 2 Lessons Learned and Relearned
Sportplane Builder 1/95
By Tony Bingelis
Here's why surface preparation is so important . . .
Paint will only color the surface to which it is applied. It will not hide:
1. Rivet heads - especially the pop rivet variety
2. The slightest scratch or scuff mark
3. The smallest dent
4. The tiniest of pin holes in a fiberglass component
5. A bump, or any kind of surface roughness
I must admit, however, with some chagrin, that I apparently have trouble convincing myself this is truly so. It is only when I carefully examine a freshly painted surface and find small areas where my preparation work wasn't all it should have been that the lesson sinks in. My most persistent fault is in not taking the time to sand away the faint sandpaper marks left by coarser grades of sandpaper. Don't be guilty of making the same mistake.
Discipline yourself to sand away all signs of surface roughness and scratches with progressively finer sandpaper. Finish your surface preparation using #350 grit wet/dry sandpaper . . . #400 grit is even better. Neither grade will leave visible sanding marks. Incidentally, using the fine grit sandpaper with water allows it to cut better and keeps the sandpaper from clogging.
I say, again, if you want a good paint job, you must remove all the dents, scratches, pin holes, and other imperfections before you pick up the spray gun and shoot the first coat.
Unfortunately, pre-paint preparations, up through the primer coat, are quite demanding and entail a lot of hard work. You will find, for example, it takes up to 50 times longer to prepare the surfaces than it does to spray on a color coat.
Although each type of aircraft construction may require preparation methods that differ somewhat, they all have this in common. Each type of construction - be it metal, composite, wood or tube and fabric - requires many more hours to be spent in surface preparation than in painting. Many builders do not realize that the prefabricated polyester fiberglassed components they receive will continue to shrink for months after manufacture. Consequently, you can generally detect the weave pattern of the glass cloth through the gel coat. This pattern will also show through the finish color coat, especially on your cowling, although it won't be as obvious to the casual observer. If this bothers you, minimize or eliminate the textured appearance by giving your fiberglass parts, particularly the wing tips and cowling, a couple of fill coats of a good sandable primer. Sand between each coat using a medium hard rubber sanding block to back up your #350 or #400 wet/dry sandpaper.
Incidentally, sanding with straight line strokes is not the best way to do it. You will obtain much faster and more uniform results if you sand in a circular motion. After you have completed all sanding preparations, blow off the sanding dust with an air hose. Poke the nozzle into all the corners. If you don't, any entrapped dust will be raised by the spray gun air blast, and some of it will settle in your finish.
Finally, examine every square inch of the newly prepared surfaces. Pay close attention to all the edges. These should be sanded or filed smooth and slightly chamfered. It is along these edges where there may be some overlooked felt pen markings. All marks you find must be completely removed using plenty of thinner. An overlooked ink mark notation, or ink line, will surely bleed through your nice finish and will cause considerable regret later.
What To Do About Pesky Pin Holes
Only fiberglass parts are capable of producing such an obnoxious blight in a finished surface. Pin holes show up most in fiberglass layups, and especially in factory made components when some of the gel coat has to be sanded to the bare fiberglass surface. Spraying primer, or paint, over pin holes is futile because the paint merely straddles the tiny holes and the pin holes become even more visible in the final finish. The cure for eliminating pin holes is to fill them by using 3M Spot Putty, or a similar automotive filler (personally, I like "Marine Tex"). You can also brush a sandable primer over the holes and rub the stuff in with your fingers until they disappear (I mean the holes). After the pin hole filler has dried, sand the area smooth. You may find that a second treatment is often necessary after you inspect the sanded results.
Unfortunately, pin holes are not easily spotted until the surface has been primed. Even so, you might miss quite a few pin holes that will, to your disgust, show up in the finish coat. What can you do when more pin holes show up in the finish coat? That's right, you'll have to live with them unless you have the resolve to re-treat and refinish the part.
The Art of Masking
If you want a quality paint job you must be willing to use quality products. I believe the tapes best suited for masking color line separations is 3M's "Fine Line Tape" or a good grade of Mylar filament tape. They produce masked lines so sharp that trim colors rarely bleed under their edges. The tough fine line tape with its polypropylene film backing cannot be torn by hand. It has to be cut with a razor or scissors. The 1/2" width is the most practical to use.
Another tape that I find to be excellent for trim striping, and for use against Plexiglas(tm) windshields and canopies is the 3/4" wide black electrical tape. It is quite inexpensive and, being very thin, can be easily stretched around sharp curves. It, too, leaves nice clean trim lines after the tape is removed.
On the other hand, the ordinary 3/4" wide (crepe) masking tape most of us buy because it is inexpensive is not as effective to use for striping because paint will sometimes bleed under its edges and leave a fuzzy looking trim line after it has been removed. As a rule, I never use the ordinary crepe masking tape against a finished surface. Instead, I use that kind of tape primarily for attaching the masking paper to and along the fine line tape color separation striping. Both the fine line tape and the electrical tapes are easy to remove. I can't say as much for the crepe masking tape which is often difficult to peel off if left on too long.
If you have to touch up or refinish your instrument panel, here is the easiest way to mask it when the instruments are already installed:
1. Cover the glass face of each instrument completely with strips of ordinary masking tape. You can do the job quicker if you use masking tape at least one inch wide.
2. Run your fingernail around the perimeter of the glass face to seat the edge of the masking tape tightly.
3. Next, using a single edge razor blade, run its cutting edge around the instrument glass.
4. Peel away the excess masking tape leaving the instrument face completely masked and the panel is ready for painting.
Obviously, it would be much easier to paint the instrument panel before installing the instruments as no masking would be necessary. However, during the installation of the instruments and other cockpit work, the day may come when you need to refinish the panel without all the work involved in removing the instruments.
No matter how careful I try to be in masking the various components, I sometimes find a spot or two where the paint has bled past the masking paper and tape. The blemish is usually a small spot or a triangle of fresh paint that somehow leaked onto the primary finish I intended to protect. Unfortunately, the discovery is made only after the masking paper and tape has been removed. The best way to prevent that sort of thing is to carefully examine every single inch of the masked edges to assure yourself that the masking paper covers everything and that the tape is down tight. Assure yourself, too, that all overlapping edges are taped tightly. Cut the tape with a razor if necessary to make the tape behave.
Your Plexiglas(tm) canopy can't stand much abuse. Keep it and the windshield completely masked, both inside and out, while any kind of painting job, no matter how small, is being done anywhere near it. This will protect the Plexiglas(tm) from accidental overspray . . . and save you from a lot of cleaning and polishing later. As you may have noticed, I use a variety of "masking" tapes.
1. The most important is the 1/2" 3M Fine Line Tape. This is the tape professional painters use to outline the color separations or trim design. It is affixed directly onto the surface you want to mask. Cheap masking tape is O.K. to use but it must never be used in direct contact with a finished surface. Instead, use it to attach masking paper to the fine line tape already in place.
2. A third tape I use often is ordinary black electrical tape. It is theonly kind of tape I ever use to mask off windshields and canopies.
Because its edges are extremely thin and smooth, laying out clean cut curved lines are possible when you use it. It is easy to apply and just as easy to remove without leaving an objectionable residue. Unlike ordinary masking tape it may be left on for a fairly long time (a week or two) without becoming difficult to remove.
Masking Paper
The best masking job results when you use regular heavy duty masking paper, not old newspapers or plastic sheets. Be sure the masking paper you use will not allow the paint to bleed through it. If it does, you will have one big problem, amigo. I don't recommend using large plastic sheets as asubstitute for masking paper. You may overlook covering all the areas you want protected because the plastic is transparent and you may not notice places left unprotected by the plastic. A roll of 14" wide masking paper is not too costly when you consider it may save you from a botched up paint job. Using large sheets of plastic to supplement the areas properly covered with masking paper is O.K. but be absolutely sure the loose end of the large plastic sheets are taped down to prevent breezes from blowing an unsecured corner up into your freshly painted surface.
It happened to me once. Consider yourself warned, so don't blame me if it should happen to you, too. Before you paint, go over every inch of the masked line, running the back of your finger nail alon the edge to be sure it is down tight against the surface to be painted. Masking paper can be removed in an hour or so after painting. Do it carefully and be sure you don't allow some of the masking paper, or the loosened tape, to accidentally contact the fresh paint.
I like to remove the masking tape and paper before the paint dries hard overnight. That kind of delay in removing the masking tape may result in a fuzzy brittle edge between the two colors.
Painting Traps Can Be Avoided
Failure to read and follow the paint manufacturer's instructions often causes the most grief for th new builder. Follow the manufacturer's mixing recommendations by carefully measuring out the correct amounts of color, reducer, and hardener as instructed. The most common finishing flaw, other than runs, is the orange peel appearance of an otherwise nice smooth shiny finish. It is usually an indication that the paint should have had more reducer (thinner) added to allow it to flow out better.
The risk, however, is that thinning paint too much (especially in cooler weather) will increase the risk of producing runs. Of course, when temperatures are high, more thinning should be anticipated. Your last act before pulling the trigger on the spray gun is normally a quick wipe of the surface to be painted with a tack rag. A tack rag is a very inexpensive, sticky-feeling type of rag especially treated for a very special purpose . . . to pick up dust. It is the one sure way to wipe off those last minute dust particles that may have gathered on your surfaces.
Painting vertical surfaces requires the greatest care as making one pass too many with the spray gun will result in runs or sags . . . especially in corners and at lapped joints, or where you inadvertently let the spray gun get too close to the surface. And yet, if you don't give the surface at least three good passes you may not get the flow-out and gloss you want.
At any rate, as soon as you have completed spraying a component, whenever possible try to turn the part you have just painted to a horizontal attitude to lessen the risk of paint runs. I hate to say it again but paint runs are prone to occur when the paint is thinned or reduced too much. And yet, if it isn't thinned enough you may get that "orange peel" appearance in the finish.
Don't be in too great a hurry to wipe away a small paint sag or run. It might level itself and become inoffensive after the paint cures . . . especially if the surface can be turned level immediately after it has been painted.
Inspect the Finish For Flaws
After you have finished painting, inspect your work closely. Hopefully, you won't find any, but here are some flaws to look for:
1. Small areas that didn't get painted, or didn't get enough paint.
2. Places where the paint leaked past the masking paper or masking tape .. . usually where there is a gap between parts or where there is a lap joint.
3. A place where you may have, inadvertently, brushed against the freshly painted surface with the spray gun, the hose, or your sleeve.
4. Spots or small areas where the paint just seemed to separate and refused to cover. Most likely that particular area was contaminated by oily fingerprints, or wasn't properly cleaned and "degreased."
5. Any localized runs, curtains and sags that you didn't notice before. These occur most frequently in corners, joint laps and on edges.
6. Felt pen or "Sharpie" ink marks that you overlooked removing completely.
You didn't find any? Congratulations!
Source Information
Marine Tex is an excellent two part spot filler that will not shrink. Most marine and boat sales outlets carry it.
