Part 1: Latex UV resistance, pros & cons
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.
In Part 2, I'll describe how I actually applied the latex paint to the Aircamper.