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Building Tips and technical articles. => Paint and finishing => Topic started by: kenneth cook on October 10, 2009, 07:57:31 AM
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I had built a speed plane using mostly mahogany and bass wood as the structure. These wings are solid and I followed up with fiberglass at all intersections and silkspan on the wing areas. I used nitrate as all filler and attachment coats. I used this mostly due to nitrate has good compatibility with just about all paints. I then took it to the shop in which they sprayed a sealer coat due to the different colors in wood that were showing through the clear coats. They typically shoot this over repaired areas to help adhesion and to use as a blocker for the base. I watched as the base was sprayed and everything was looking great. The clear went on after everything dried and it went into the paint booth for a baking. This is where some problems cropped up. The wing area developed some mild blistering and some very minor spots on the fuse. The painter was more upset than myself actually. I feel it was due to the silkspan. I more or less brushed a very thinned mixture of nitrate to attach the silkspan. I'm sure this would of been fine for a dope shot airframe but I think solvents got trapped between the wood and the underside of the silkspan. I believe the baking caused the solvent to escape causing the blisters. . When the majority of you that use 2 part auto clear, do you bake the finish? I know for a fact that this not only levels the surface but readily speeds up the drying and hardens the surface. I've used 2 part auto clear but not to any great level. I used it mainly within the engine compartment for fuel proofing purposes only. In addition, would I have been better off just to use fiberglass resin on the solid surfaces? I did fiberglass over the leadout tubes that were within the wing. I was amazed at how much the material sucked back into the groove. The plane spent approx 45 min in the booth at about 130 deg. Ken
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Well, if you don't know how to paint your plane, I wouldn't expect the local fender man to know either.
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Just as a casual observer, I would theorize that oven-baking a wood model is risky bidness.
Not that you baked it hot enough to start a fire, but a wood structure is bound to have some air or maybe residual thinner trapped inside. I would expect the expansion to make it literally explode it's way to freedom. This wouldn't happen with a solid metal part.
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Yea, Ty has it. Problem I have is humidity level. Takes forever for stuff to dry when the humidity is 80%. A low temp oven with a dehumidifier would be nice.
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There is really no reason to bake the clear when you are done spraying it. the two part i always use "transtar" dries to the touch in about 60 minutes and is hard over night and levels quite well, if you need it to dry faster most shops have what they refer to as "spot clear" or "touch up clear" and it dries to the touch in about 20 minutes and is hard enough to handle or buff in 2 hours. it doesn't tend to level quite as well but it still works fine.
in either case i think the toughness gained by baking isn't neccesary on our models.
Dave jr.
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Having some experience in this process the short answer is yes, baking it caused the grief. Lawuer by nature is virtually NEVER dry. It dries and shrinks for years. Hence the reason it is so repairable. The problem is in fact that this process occurs. When you bake it, the Urethane clear kicks, and seals off, then the solvents in the laquer want to escape and have no path to do that, ESPECIALLY on a solid wood model like a speed plane. On a built up plane the solvents can find an escape path through the fuse or wing and out the leadouts etc. Still baking is not a good idea.
Of course, there is another option, that being that after every layer of differing material, you go bake it. so you build up the substrate of laquer, then bake it, do the glass , then bake it, color coats , bake it, clear, and bake it. Were it me, and I felt it neccessary, I would start with a bake cycle of say 150 degrees, then for each step, cut the temp by 10 degrees . In composites this is how Post curing is done. you bake it to a temp higher than it will see in its real life and then it is stabilized for any temp under that.
Now for the clears, spot clears are really no different than overall clears after a period of 24 hours or so. The real attribute to spot and repair clears is that they tack off faster. IOW, they are dust free much sooner than overall clears and this allows the painter to cycle cars through the booth faster, ( or in most cases these jobs are painted on the floor or under a prep station where dust intrusion is a problem)
Baking doesnt really create a stronger finish long term it pretty much just accelerates the reaching of that point. Its a chemical reaction and as long as all chemicals are together in the proper balance, it is going to cure to the same hardness.
I dont really like to bake clear unless its a deadline thing personally. I dont like to reduce my window to polish that way. For those who havent, rubbing and polishing urethane clears is best accomplished after about 24 hours, and before 36 to 48 hours has passed.
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>>Lawuer by nature is virtually NEVER dry<<
I agree. Lawyers are never dry. Oh, wait, you meant Lacquer. It makes so much more sense now. 8)
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I have a stunt plane painted with acrylic lacquer with a two-part clearcoat over it. It has some black places that still get bumpy a year after I painted the plane when it sits in the sun awhile. The bumps go away when I sand with 2000 paper and buff, but I have started to sand through the clearcoat in places. Mark explained the solvent trapping to me and added the wisdom that "black lacquer never dries." I am looking at pastel colors for the next dog.
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Howard,
A nice pastel pink with a nice mauve trim would be great. OK, Maybe not so much.
Go Huskies!
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Once again, incompatability of materials. How many times has this come up? With the same answer? Once again, matched finishing systems, Mark has explained this thoroughly in another thread. I am not a painter by trade but was taught by a professional. That's how I know. Where on the can of nitrate does it say " spray car paint on top " ? HB~>
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I've one numerous finishes with dope up to topcoat then catalysed polyurethane. If done correctly, it isn't a problem. It can be tricky if you have to repair the finish, but it isn't black magic. My classic plane, sitting even as I type this, down in a friend's house in Tucson, was finished this way and sat in the 90+ temps without problems. Of course, there's no black on it.
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I like solid wood for the smaller racing planes and have quit using dope on them. The procedure now is water base poly clear sanded. Then if I want clear finish I go to the clear coats. Of course sometimes it takes several coats to get it like I want it. Mucho easier with colors before the clear. DOC Holliday
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I think a good part of the problem had to do with the "oiliness" of the wood itself. Finishing over freshly worked (machined, sanded, etc.) surfaces can often cause an ugly, orangey "bloom" under a clear finish. The only thing I can suggest is repeated wipes with an alcohol sponge, followed by moderate baking (like in a non-air-conditioned garage) for several cycles. This, followed by another alcohol wipe (I'm starting to sound like "Monk") and thorough drying, but without the bake, which could bring still more resin to the surface.