Painting aircraft with latex paint

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.!

If you haven't been there already, Part 1 of this article discusses some pros and cons of using latex for aircraft paint, as well as a description of an accelerated aging test I conducted on Stits Polyfiber with the Sherwin-Williams paint.

5/11/02 Update

Paul Yarnall sent the following message to Tandem Talk:

Hi Everyone... My airbike partner and I are just finishing up an airbike... We covered using the AFS system of water based glue and UV barrier... great stuff to work with... and plan to shoot the color this weekend in Sherwin Williams latex. We had done a bunch of test panels in latex and we were getting pretty discouraged since none of the techniques variously described using foam brushes or rollers produced a finish we were willing to commit to our beautiful wings. When we dumped the Floetrol, reduced with 20% water and applied with a $60 Harbor Frieght HVLP conversion gun, the results were fantastic. We have attempted to abuse the latex panels and there is little doubt as to the bonding of the latex to the AFS Cecofill... they seem to be very compatible.

I'm very interested in the long-term success of the latex paint over the Cecofill, and hope that Paul will stay in touch and let us know how it performs over the years. This seems like an ideal solution if the latex paint will stay firmly attached to the Cecofill through years of temperature/humidity cycles. We also need to know if spilled gasoline and/or oil will have any deleterious effects on the bond between the two components. I'm hoping that it works great, Paul.

2/9/03 Update

Paul sent the following additional details about his experiences with AFS products and latex paints.

Hi Doug....

Sorry to hear about your unhappy results with the AFS products. I don't recall if I had mentioned it before, but we had intended to use the AFS for the topcoats but that turned into a disaster and we washed it off before it cured... that's when we started on the latex quest.

I am surprised that you also had problems with the CecoFill. I can say that I have never tried spraying it. I seem to recall that even AFS recommended brushing it on, (but don't hold me to that).

(ed. note: The CecoFill bottle says "For best results we recommend spraying." It does later say that it can be brushed or rolled, though.)

The tool of choice is the throw away foam brush found in every hardware store. I think the brush method forces the cecofil into the weave and overcomes the surface tension that aggravates the flowing out we are used to seeing with solvent based coatings. I have been using a water based clear polyurethane for some wood working projects... the only way I can get a nice wetted out surface is by spraying (same HVLP gravity gun I used on the Airbikes) but if the spray coat is too light it doesn't flow and will display a classic "orange peel" effect we use to associate with silicone contamination when spraying lacquers and enamels. You would swear that the surface was contaminated but if the coat was applied a little heavier it would flow on very nicely. Also the tiny holes in the wood grain is very resistant to filling and flowing over ... just like the weave in the fabric.

You may have no desire to see or smell the stuff again, but you might find it instructive to try a few test panels with a foam brush. Even without sanding, the brush marks are very subtle after it is completely cured.

As for filling in the blanks in my own latex experience that you requested, I was able to piece together the following... (thought I had written some extensive notes but can't find them)...

On a CecoFill prepped surface, we applied Sherwin Williams A100 Exterior White Latex Primer, their part number 600-0483. We reduced it with water. Sherwin Williams says not to reduce with more than 5% water. I think we went a little more than that, but I did write down that it was 125 sec. using a viscosity cup. We used the $50 gravity feed HVLP "conversion" gun from Central tools. A terrific value. It sprays better than a $700 Croix gun I used for years until I tried this.

As I said before, my partner and I barged ahead and had some Sherwin Williams "Exterior Accents" 25 year paint mixed before we experimented with it. Big mistake. No combination of water, pressure, Fleotrol, cursing or arm waving would make that stuff spray a finish we could stand to look at. We tried some others to the same result. In desparation we went down the paint isle at our local home center reading labels to find something else to try. We took home a quart.. . (we got smart and stopped buying gallons)... of Dutch Boy, Exterior Door & Trim, Acrylic Latex, HI-Gloss Enamel. The reason I decided to try it was the factory allowance for spraying of up to 1 pint per gallon of water... 12.5%. Spraying this stuff was as nice as the other was bad! It saved our bacon on this project. In gallons the neutral base is part number 16-809. Tint to your color of choice.

As I said before, the latex shows some swelling when exposed to fuel, but seems to do no permanent harm. We elected to make our wing tanks removable by putting an aluminum tank cover on just as you would do on a "real" airplane. Minor fuel spills evaporate before it reaches the fabric most of the time. We also are using only straight pump gas with no oil so any spills don't carry any oil to the fabric either. (We converted both Airbikes engines (one 377 and one 447) to oil injection. We also converted both to "free air" cooling. Together these changes have made for very happy engines and much happier pilots, NO MIXING!!.)

Back to the spraying... despite unfortunate experiences such as your own with CecoFill, I still consider it a great filler/UV barrier that is very easy to use and is virtually fume free. I have used it on real airplanes and on the Airbikes and I whole heartedly recommend it, BRUSHED on. The Sherwin Williams A100 went on beautifully, covered well and is a great base for any other top color. The Dutch Boy did the same for color. But for anyone contemplating a "rubber skin" airplane, throw together a dozen 12" x 12" square wood frames, staple on some scrap dacron, shrink it per the usual way, and try all the materials and methods you want to use before attacking the airplane. You can work out the best thinning ratios, best spray settings and techniques and so on. It is worth the few hours of extra work. Once all the variables are nailed down the entire Airbike can be painted in two days.. one for the white and the color the next day.

Doug, I have attached a picture of our "bikes". There is a couple of feet of snow in front of both doors so this is the best I can do for now.

This is about all I have to offer on latex and Airbikes, but I would be happy to address questions. Doug, you can use any of this that you wish and you can edit it to fit your space as you see fit.

Sincerely,

Paul Y.