Painting the wings [120.0 hours]

I haven’t posted for a long time, but have been pushing along various jobs including the baffles, wing root covers, fuel system and painting various small parts such as the flaps, ailerons etc. While the wings were still pinned on to the fuselage, I did a complete fuel system and tank test, by filling the wings with fuel and running the fuel pumps each side through the entire distribution system, except for the final lines to the injectors. No leaks, good fuel pressure, no weeping rivets. After this, I took the wings off to paint. Since this was a major activity I’m posting some details here.  If you’re not interested in the details, pictures are at the end of this post.

I thought about how to set the wings up for painting quite a bit, and finally decided to make a set of stands to hold the wing in place vertically, with the nose highest. I discounted more complex schemes such as rotisseries, RV-10 wings are quite large and a rotisserie would need to be quite robust. Apart from which, it is unnecessary. Painting the large wing surfaces vertically gives you the easiest access and optics to maintain a wet edge, and in an amateur setup without highly filtered air it exposes the minimum area for dust to “settle” while the paint is drying.

I was going to build a booth out of structural pine and plastic, but after pricing the required material I elected to save the hours and buy a cheap inflatable booth. It’s large – 10m x 5m – and the blower to keep it inflated is noisy, but it was easy to set up in the hangar and the filters were quite adequate.

I put a lot of thought into how the stand works, and then built it out of scrap material. There is a stand for the wing root (main spar), a stand for the outboard (light) end of the wing, and an intermediate piece of stand which allows me to support the wing after the white is painted, so the end stand can be temporarily removed and the wingtip fitted for taping. The two ends of the stand are tied together with a pair of 75x35mm pine pieces, 4 metres long, and the resulting “channel” can be lined with plastic to catch the water runoff from the acid etch and rinse operations. This was another reason for doing the wing vertically – I could do every operation from acid etch to final paint in the same stand, with no changes apart from the switch while the wingtip is temporarily put in place for taping.

I painted one wing at a time. Spraying takes next to no time at all, but taping off takes many hours and I didn’t want to have to do this for both wings at once. I did the left wing first, then after learning a few things did the right wing slightly differently. It took 3 (long) days to do the right wing, from start to finish, with four colours. Here’s the order of events:

1. I scuffed the top side of the wings while they were on the plane, and the bottom side of the wings on a pair of workbenches after they were removed. I de-greased both sides at this point, so that they were “substantially” clean before moving the wing into the booth. I also taped up both ends, all the inspection plate holes, fuel drains etc. at this stage.
2. Tape drop cloths to the floor of the booth. Tape builder’s plastic to the water channel, making sure it is pressed down inside the channel. Use minimum tape, because it has to be taken up again while the wing is in place.
3. Move in the wing stand and screw each end to the channel pieces.
4. Move in the wing. It’s a two man operation to lift the wing off a pair of workbenches and onto the stand. The poor guy holding the outboard end has to hold it while I secure the main spar. I inserted 3/8″ teflon tubing into the outermost 3/8″ holes on the spar connection, and used batton head screws with washers (and ample layers of masking tape) to secure the wing root in place. Then I move to the outboard end, hold the wing up, and the other guy can release the wing and insert the support for the far end of the wing, clamping it into the stand and screwing down a holding cap. The support piece has padded surfaces for the spar, two of them separated by about a 2 inch gap, for the outermost two bays of the wing. With this support in place, I can release the weight and the wing is fully supported, and moreover is fully exposed to be able to spray all surfaces.
5. Do any remaining tape up for paint. Fuel cap holes, and a few other bits ‘n pieces.
6. Fully degrease the wing. This is easy since it was mostly done before moving the wing into the booth. Don’t forget to pay special attention to the skin overlaps, where grease can stick to the edge.
7. Tape up the seams so the acid etch doesn’t penetrate into the wing interior (much). Tape over the fuel cap holes, and the the inspection cover holes etc.
8. Do the acid etch. I used cheap 2 litre and 5 litre garden sprayers, the type you pump up to spray. The 2 litre container was for Alumiprep 33 solution (about 50/50 with distilled water), and the 5 litre sprayer contained distilled water. Spray the acid at low pressure horizontally across the wing – it’ll all run downwards – and catch it with the 3 inch brush using horizontal strokes. Wear safety glasses. Work your way from the top to the bottom. I did half of the length of the wing at a time, front and back (spraying near the top oversprays to the other side), twice, allowing time for the acid to work, then rinsed it all off with the 5 litre sprayer, never allowing any of the surfaces to dry. The first acid you spray on will “break” and quickly run downwards in trickles, which is one reason you need to keep brushing horizontally to spread the acid around. Once it has had a chance to work, the water will not “break” much or at all. With one half of the wing done, front and back, I switched to the other half. I kept going back and spraying water over the first half, to keep it all wet (oxide won’t grow back under water). Once the wing was all done, the channel was full of acid + water runoff, the entire wing surface was wet, and there was also a bit of water lying around outside the channel (inevitable) but not much. In total I used about 1.5 litres of acid+water solution and 5-6 litres of rinse water (I had to refill the container once, a 10 litre sprayer would have been better). Now let it all dry.
9. After 5-10 minutes, I removed all the wet tape and rinsed around the exposed areas where a bit of acid had penetrated in and around the tape, using a bit more distilled water in a small squirt bottle.
10. Let the wing dry. I used some cleaning cloths to run along the bottom where the skins run behind the aft spar, since water tends to accumulate at this bottom (aft) edge and take a lot longer to dry. Heaters may be necessary if the weather is cold. Hot days are best, get the thing dry as quickly as possible because the oxide is growing back…
11. As soon as the wing is dry, spray EAP-9 (or your adhesion promoter product of choice). You only need a very fine mist of the EAP-9 product. Allow to dry.
12. Move in the wingtip. Spray the primer on both the wing and tip. I used PPG CA7700B primer, as part of the system I used. The primer only needs to be sprayed on as a very thin coat – almost translucent. Allow to dry.
13. Remove the water. You can “roll” the plastic from the outboard direction, then using a small cup and a bucket, get all the water (and primer overspray, and acid) out of there. I did this later for the left wing, but it’s best to get it all out of there asap because it’s a mess. Be careful not to splash any of this toxic mixture on the wing.
14. Wet down the floor (it contains a lot of primer overspray) and spray the main topcoat (in my case, white). I used PPG CA8800 paint. It’s expensive paint, but easy to use and will outlast me. I sprayed two coats using the CT2 thinner, 40 minutes apart. It took me 15 minutes to spray the entire wing and wingtip (I’m slow at this), so plenty of time between coats to clean the spray gun and mix up the next pot of paint (induction time is zero). The coats need to be thin, this isn’t a truck. Although it isn’t necessary to clean the entire gun between coats, I’ve always done so. Hint: After the first coat (only), as you finish cleaning each part with gun cleaner, rinse it in Acetone. This rinses off the gun cleaner, which can be a little slow to dry (in my case), and the Acetone will evaporate quickly. That way, when you reassemble the gun for the second coat about 20 minutes later, all the parts will be bone dry.
15. As soon as the white topcoat is “dry to tape”, or a bit more, re-configure the stand so the wingtip can be temporarily fitted. This step can be skipped depending on your paint scheme. In my case I had fairly acute sweeping angles of grey accent paint crossing the wing-wingtip boundary, and I wasn’t confident of getting these right without taping up the entire shapes with the wingtip in place. The mid stand piece I made was secured to the bottom channels, the outer most flap support, the wing tie-down hole, and the aft spar near the inboard aileron hinge bracket. I’ve included pictures showing how this was all done. I used a piece of 3/8″ fuel line inserted into the flap bracket hole to protect it from the screw, with plenty of soft (baffle material actually) padding and washers to spread the load, and of course masking tape.
16. Now is a good time to tape down some new drop cloths on top of the old ones, at least in the outboard half of the wing where you’ll be mostly working from now on. If you have a poor man’s spray booth like mine.
17. I made some patterns out of brown paper (and tracing paper) for where the various colours go. It’s a lot easier to do these with the wing on the plane, or the bench, and without the time pressure of spraying paint. Using the patterns, I taped off near the boundary of each colour. I use 3M 233+ (green) tape for anywhere that tapes on to freshly painted surfaces, and cheaper tape anywhere I’m taping down onto the top of the green tape, or other covering material. Once the basic shapes are outlined in tape, carefully slice the tape with a sharp knife on the wing/tip split line, and remove the wingtip. Re-configure the stand back the way it was (to make it easier to cover the rest of the wing up for protection from overspray.
18. Tape everything up. This is the part that I found takes a long time. Tape things up so you can easily remove tape covering successive colours, in the order you’ll spray them.
19. Tape up the edge(s) you’re doing, in my case the second colour to spray was blue. I use 3M 471 vinyl tape for the edge work. It’s best to avoid crossing rivets or seam boundaries, but in some cases this is inevitable.
20. Clean the exposed (white) surface with isopropyl alchohol. It’s best to avoid touching any surface at all, but I can’t manage the taping up wearing gloves so it’s inevitable to get a bit of a touch somewhere and this has to be cleaned up. Press all the blue edge tape in.
21. Go mix the paint pot. Spray the colour (again, in my case, two light coats 40 minutes apart). the very last thing I do before spraying the colour is to re-press all edge tapes, with a gloved finger. A final wipe with a tack cloth is also a good idea.
22. At the “correct” time, peel off the edge tape. It should be prepared so as to make it easy to peel off, and the direction should be correct (towards any point in the pattern). In my case I found 75 minutes after the second coat was about the right time, at 20-25 degrees ambient. Clean up any paint that (for example) ran around a rivet the edge crossed, using a Q-tip and thinners. It’s far better to deal with any artifact now with a Q tip than later with an air brush. In some cases the air brush will be inevitable.
23. If there are more colors, once the paint is dry-to-tape, rearrange the taping and repeat the above process for the remaining colours. I was able to spray two colours per day, white+blue on day 1 and dark+light grey on day 2.

It was quite an effort to do all this for two wings, but I’m happy with the results. It’s an amateur job of course, but close enough to what a pro would do at a much cheaper cost in labour. The paint I used is buffable, if I want to work on any areas later. There is a buffable clear coat available for this paint system, but I elected not to spray the clear, due to the extra expense, weight and the fact that I’ve never successfully sprayed clear coat before and didn’t want to add more work to the job.

I’m having a break from the project for a few weeks, before getting on with preparing and painting the fuselage.