The following was taken from http://www.questiongravity.com/airbike/latex1.htm
In the spring of 1995 I completed this Ragwing Ultra-Piet (3/4 scale Pietenpol Aircamper) ultralight, using Sherwin-Williams Acrylic Latex over bare Stits Polyfiber fabric. I’ve received a lot of requests over the years for information on my experiences with the relatively new “latex process,” so what follows is what I know from my experience painting and maintaining one airplane. I hope these pages are helpful to you.
Latex paint is very flexible, and therefore well suited for use on unsupported fabric surfaces that will inevitably see flexure in normal flight service. You can wad a piece of latex-painted fabric into a tight ball, and then lay it out flat without any seeing any cracks in the paint (I’ve done it — it’s very convincing). Latex is also very user-friendly, locally available, and dirt cheap compared to “real aircraft paint” (should easily cost hundreds of dollars less to paint an entire airplane).
It is generally recommended to apply some sort of UV barrier below the color coats, and most builders that use latex seem to comply. The most common approach seems to be initial coats of flat black latex, thick enough so that direct light does not pass through it. Then the color coats are applied over the black. Makes the innards of the airplane kinda ugly, I hear. I opted to not add a UV barrier for two reasons. One, I could tell that my Ultra-Piet was already in danger of exceeding the Part 103 weight limit and I wanted to keep the paint job as light as possible (later found to be a very wise move). Two, I didn’t feel that a UV barrier was really necessary. If I was still flying the airplane in 10 years, it would probably be ready for new fabric anyway, I figured, UV degradation or no.
To test my theory, I decided to subject a sample of latex-painted fabric to an accelerated aging test, something much harsher than it would see on a hangared (UV-sheltered, for the most part) airplane. So I made a simple wood frame, attached a scrap of the Stits fabric with Poly-Tac around the edges and heat-tautened it. I cut a patch with pinking shears, glued it to the fabric, and cut a square hole through both layers to represent actual patches in the Aircamper around the cables and struts that protrude through the fabric (leaving the fabric unsupported around the hole except for the patch).
Here’s an “after” picture (sorry, I failed to take a “before” picture.) In the upper right corner you can see one brushed layer of red latex (wish I would have added a second coat of red). The balance of the sample was brushed with white latex — one coat over the entire surface and a second coat in the lower left quadrant. (The stains at lower left demonstrate the effects of canine urine on latex paint — I told you this was going to be a harsh test!)
This panel was leaned outdoors, against my shop, for the next 6 1/2 years, facing south in direct sunlight and directly exposed to the elements. I just brought it inside a couple of weeks ago as it is only now clearly non-airworthy. Every few months I would inspect the panel and try to poke a hole in it by digging in hard with a fingernail. Somewhere around the 6th year, I heard the fabric crinkling during the fingernail test*, although I was unable to repeat this later. Up to this point, I would have flown the material, but now I had to concede some doubt. (*I now think this was the paint cracking at the edge where the fabric was being stretched tight over a corner of the wood frame, and not the fabric itself tearing. I know how to duplicate this sound now, and it seems to be paint-related, not fabric-related.)
Today, after 6 1/2 years, my thumb went through the material during the fingernail test (you can see the new hole in the upper right corner). It should be noted, however, that the material failed in an area that had only received one coat of paint. At this time I cannot get the material to fail in the area with two coats of white paint, even while forcing a thumbnail in with assistance from the other hand. This suggests to me** that it is possible for Stits-type fabric with two or more coats of latex paint (with no other UV barrier) to be left directly in the elements for six years and still be airworthy. (I plead ignorance of how an official fabric-testing instrument would compare to my fingernail test. I’d guess that a real fabric tester might be a bit too pessimistic for lightly-loaded ultralight use, though.) White paint is probably the worst case, too. The darker — or more opaque –the paint is, the better a UV barrier it will be, and the longer the fabric should be protected from UV degradation. (**Paranoid note to litigious society: I am speaking only for myself, and do not recommend that anyone actually trust their life to fabric and/or paint under the conditions mentioned here!)
This is a close-up of some “ringworm” cracks in the paint. This only occured in the area with two coats, and I can’t say exactly when it happened. I’d guess this appeared somewhere around 4-5 years after the test started. I’d also venture a guess that the panel was about -10°F at the time the cracks were formed, and perhaps it was struck by hail,an icycle, or a dog! Despite some cracking, the paint overall still has excellent purchase on the fabric and will not peel or flake off. When the paint is applied in a manner that allows it to soak into the fibers properly (see Part 2), this seems to produce an excellent and long-lasting mechanical bond.
Here is the patch and hole. The edge of the hole is still in excellent condition, but on the left side of the patch you can see the Poly-Tac bond between the fabric and the patch has failed from the outer edge to the hole. The bond is quite intact elsewhere, though, so perhaps my bond wasn’t executed as evenly as it should have been.
One weakness of latex that I noticed is a slight sensitivity to gasoline. Spilled gasoline should be wiped off pronto, as it can soften and swell the paint slightly. This swelling seems to go away given time, but it can leave a slightly visible scar or discoloration.
I followed Roger Mann’s advice (designer of the Ragwing Ultra-Piet) and applied Armor All to the paint once or twice a year. Roger says that Armor All contains a UV barrier, and it also restores a nice sheen to the paint. I did this 6-8 times to the Aircamper over the years, and the effects on the paint seemed to be nothing but positive.
Another weakness of latex paint seems to be in its resistance to bird droppings. I’m no chemist or biologist, but bird droppings seem to be quite acidic and they will etch their way partially into the paint. Sometimes they’ll wash off and leave no trace, other times you’ll end up with a lightly-etched reminder of a truly ugly wad of avian excrement that resembled blackberries and cream gone bad. Nasty stuff. But I think the worst scars the airplane incurred were from bird eggs tossed thoughtlessly out of a nest in the hangar’s rafters. The goo that ran out of the shells got into the paint big-time and was very difficult to soften and remove.
I never had occasion to repair any latex-painted fabric, so I can’t comment on that. But this is mentioned in “Maximum Paint Job, Minimum Cost,” a 12/01 Kitplanes article by Kay Fellows about using latex paint on aircraft. Fellows writes, “If you get a hole in the fabric, simply cut a piece of cloth athletic tape with pinking shears. Fit it over the affected area, get out the trusty brush and house paint, and daub it over the tape. This process takes about 10 minutes and gives you more time to fly.” I have to question this suggestion, though, as my experience with athletic tape is that it is low in tack. I’d be more interested to know the results of sanding the old paint around the hole and attaching a Polyfiber patch with Poly-Tac. If the Poly-Tac doesn’t react negatively or bond poorly with the latex paint, that should create a much stronger patch.
Fellows seems to agree with me on fabric life in general: “As we all know, after 1000 hours or 10 years, you should recover your plane and give it a good once over. I would rather pull the fabric off my plane knowing it only cost $200 than if it cost $1000.”
I’ve concluded that for a low-cost and low-hassle homebuilt airplane, latex paint is an excellent alternative to the high-dollar aircraft paint systems out there. It may not give you a grand champion award at Oshkosh, but properly applied it looks nice enough to satisfy even me. I further conclude that a UV barrier is not worth the cost, hassle, or weight on a hangared airplane (especially on an ultralight). My gut feeling is that a latex-painted airplane that spends most of its life shielded from direct UV rays (under a roof) will remain airworthy and attractive for at least 10 years, and for the money, that’s good enough for me.
Part 2: How I applied the paint
I had the good fortune of working for an employer with a temporary excess of warehouse space, which they let me use to apply the fabric and paint. This would have been really difficult in my crowded and dusty shop. I made special sawhorse caps lined with felt for resting the wing panels on. In the back of the photo you can see me setting up a spray booth with PVC tubing and plastic sheeting (worked fine for me, but be sure to read the caveats in Sid Lloyd’s Fabric Covering and Painting page. 2/9/03 note: See photos of my new metal conduit version here). The booth had a roll-up flap for a door on the wall by my left arm, and was vented by a common window fan and furnace filters taped into the opposite wall. Don’t worry about using a spark-free fan, because latex vapors are not flammable! (Non-flammable, inflammable, non-inflammable — don’t get me started. It won’t burn, dammit.)
I’ll never regret taking the time to build this “rotisserie” for holding the fuselage. On the front you can see a plywood disc with a lock-pin hole every 45°. The fuselage could be rotated to the best working angle for the task at hand and then locked in place with the black handled pin (not being used in photo). This made a potentially back-breaking job about as effortless as it could be.
After applying the fabric, tapes and reinforcing patches with Poly-Tac and heat-tautening with an iron per the Stits instructions, I applied the color paint directly to the bare fabric with no Poly-Brush and no UV barrier (see Part 1). But first I scrubbed the material with a mild detergent — this may be a critical step IF your material has any glycol residue in it from the extruding/weaving process. Some builders say it’s an issue, others say it isn’t. I didn’t care to take chances.
I used Sherwin-Williams Acrylic Latex Interior/Exterior High Gloss Enamel (this was in 1995, so the name and/or product may have changed by now). The creamy-white paint for the wings and tail feathers was thinned with water for use in a standard Binks-type spray gun (didn’t have an HVLP gun at the time). I don’t remember how much I thinned it… maybe 20-30% water, but don’t hold me to that. I just thinned it until it had a viscosity similar to oil-based enamels I was familiar with. The first coat should be applied heavily enough so it will soak into and around the fabric fibers, but not so heavy that it soaks through too much and starts running on the back side of the fabric. Allowing the paint to soak in and wrap around the fibers gives you the mechanical bond with the plastic (polyester) fibers which will assure a durable, non-flaking finish. Latex paint will not establish a good chemical bond with plastics — polyester included — so a mechanical bond is important.
The thinned paint, especially being very lightly-colored, builds slowly and has a difficult time hiding contrasting features below (such as those hideous black stamps Stits puts on the fabric every few feet). I tried to get by with two coats, but it just wasn’t enough. The third coat gave an acceptable opacity and a nice semi-gloss sheen. Sure, I could have added more coats, but I was concerned about the weight of the airframe and wanted to keep it as light as possible.
Many people recommend adding Floetrol, a latex retarder, to help the paint lay out flat before it gets tacky. I’m reasonably sure that Floetrol (mixed per the instructions) gave me headaches. I would spray a panel and these greasy-looking splotches would appear, and the paint would flee the area. After trying several times, having to wipe the fresh paint off immediately before it dried that way, I stopped using the Floetrol and the problem went away. But other people seem to have good results with it (it’s mentioned favorably in the 12/01 Kitplanes article “Maximum Paint Job, Minimum Cost”), so I don’t know what to say. It seems telling that the Sherwin-Williams clerk tried to discourage me from buying it, but I had been told to use it and by golly I was gonna use it.
I brushed two coats of red onto the fuselage. The brain cells that stored what kind of brush I used are dead or dying, but if I had to guess I’d say that I tried foam brushes but switched to a high-quality bristle brush because of brush stroke problems at the edges of the foam brush. I think I do remember that the first coat (whether sprayed or brushed) should be thinned slightly so that penetration into the fabric fibers is assured. I probably applied a slightly-thinned first coat of red, and then a full-strength second coat. When brushing, the pressure from the brush helps force the paint into the fabric, so it may not need to be thinned as much as a first coat sprayed on. You can peek at the back side of the fabric while you’re painting and see what kind of penetration you’re getting. I am not aware of anyone using latex paints over fabric that has first been sealed with Poly-Brush — if you know anything about doing this I’d love to hear about it. My gut feeling is that the latex won’t get a decent chemical OR mechanical bond to the Poly-Brush, and is likely to start peeling long before the fabric is ready to be retired. I hear about lots of builders mixing and matching primers, color coats and clear coats from different systems and/or manufacturers, but I consider this an extremely risky practice. There are a lot of unsuccessful (blistering, peeling, etc.) aircraft paint jobs out there to give creedence to this opinion, too. A homebuilt airplane represents a HUGE investment of life energy and money; therefore I don’t think it is worth it to mix and match different kinds of paint and risk ruining the whole thing. (See 5/11/02 update below)
I had to use careful and consistent brushing technique to keep brush strokes to a minimum on the red surfaces. The red was a more substantial paint, and two coats produced a very satisfactory opacity, although the black Stits stamps could still be seen if you looked closely in good light. If you looked closely with a glare you could see some slight unevenness in the paint’s surface (brush strokes), but you had to be looking for them. I’ve heard a lot of favorable comments about using rollers to apply the paint, but I didn’t have enough confidence in them to try it. The Kitplanes article mentions the use of Floetrol to allow the paint to flatten out after rolling.
I remember wet sanding the paint between coats (probably with 400) to knock down the little fuzzies that got in the paint despite my precautions with the booth (I sprayed the walls and floor of the booth with water before painting, to keep dust down and also to raise the humidity level and hopefully retard the paint some.) You can bear down on the sandpaper over unsupported stretches of fabric, but be extremely careful over ribs and other frame members — you’ll sand through the fabric in a heartbeat if you’re not careful. I also remember lots of little corners on the pinked edges standing up a bit after each coat of paint. These can be ironed back down, as heat seems to melt both the latex and the underlying Poly-Tac and allows it to re-bond.
Personally, even though I am a perfectionist in many ways, I’d much rather be “up here” with a good paint job than “down there” still trying to achieve (and pay for) a perfect paint job. Also, I was soooo tired of the project by the time I got to the painting that I simply didn’t have the time or motivation to labor unnecessarily over the paint. Notice the lack of masked trim colors. K.I.S.S.!