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Building Tips and technical articles. => Paint and finishing => Topic started by: Steve Thomas on November 20, 2010, 05:03:43 AM
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Just looking for some thoughts on finishing a semi-scale Supermarine Spiteful I'm working on at the moment. As you can see, the whole thing is sheeted/planked. Normally I'd seal the balsa with a few coats of dope, apply silkspan, then more coats of dope, sanding sealer, primer, paint, etc - but is there a better way? Would it be significantly heavier to use thinned Z-Poxy to seal the wood instead of the initial coats of dope? If I do, could I get away with not using silkspan at all?
The model is not intended as a competition stunter, more as an aerobatic sport model. Span is 53", area about 560 sq.in., and power is a sidewinder PAW 35TBR which I've had sitting in a drawer and would like to see flying. I've already made a few weight compromises for appearances' sake (eg heavyish diamond-tread wheels, strut covers etc), so would like to avoid the finish being any heavier than it needs to.
Cheers
Steve
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No use trying to reinvent the wheel, I'd stay with what has worked. It looks to good to take a chance on screwing it up on something your not familier with. Go with the dope y1 y1 y1
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Steve,
You can also go with two coats of Defthane sanded between coats to seal the wood and then finish with dope.
Bob
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Hi Steve,
I was just reading Bob Hunt's article on his Genesis from 1973. He went into detail about using polyester resin to seal the wood. He was very emphatic about getting as much of the resin off the model as possible before it cured, just leaving it in the pores of the wood.
I have a model that is now about 10 years old where I used only nitrate dope as a filler on the wood. So far, no grain is showing. It is a very light finish as far as weight goes. Still, that is the only one I have done that way.
Big Bear
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Thanks guys, looks like I'll stick with the dope then. I've had concerns about shrinkage causing dips between the ribs, but I'll try to get around that by adding a bit of castor as a placsticiser.
Cheers
Steve
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Use Non-Shrinking dope on sheeted surfaces, and don't rub-down to vigorously, then you shouldn't get the lows between the ribs!
Cheers Neville
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waterbased polyurathane seals the wood and sands smooth - wood filler in any low spots -sand - then apply finish coats - REAL easy. Light and gives a good finish
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That sounds interesting, Wynn. Could you get away without using any silkspan (assuming adequate structural strength to start with)? Is some sort of sanding sealer still necessary, or does the water-based poyurethane get things smooth enough without it?
Steve
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Watch out if you are using something like Polycrylic (water based urethane). It can warp the wood if put on raw wood (ask me how I know).
The point of using resin like Bob Hunt did was to harden the wood so that you can fill the grain providing a barrier so that the wood doesn't soak up too much material and so get heavy. Silkspan does much the same thing but you get some strength out of it too. It's certainly possible to use a material to harden the wood. Thinned epoxy, resin or other material can work and I've done both. As Bob noted, you have to get as much off as possible. You just want to harden the surface. Were I doing this, I think I'd try urethane. Shoot on two part polyurethane clear, wipe off as much as possible. Once it hardens, sand then shoot a catalyzed primer like K-36 to fill the grain then shoot urethane auto colors and a catalyzed polyurethane top coat. Few coats so pretty light. Easy and should look good.
But as others have said, probably better to go with what you know for this one and build some easy profile to experiment on.
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Watch out if you are using something like Polycrylic (water based urethane). It can warp the wood if put on raw wood (ask me how I know).
True Randy - but if you dont go too heavy with it - it is not too much of an issue - especially over foam cores.
You can also use it with .5oz glass cloth for a smoother finish - makes a tough skin to get through as well.
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My next weapon will be fully sheeted, no open bays,, ( well excepting the engine compartment with the hatch removed,, ) H^^
I plan on using light tissue, and catalyzed Urethane clear as a base to harden the surface. My PLAN is to do like the old glass method of sticking it down, then blotting off the excess. This will be lightly sanded after curing, followed by one coat of K38 catalyzed priimer. That should see the project ready for paint assuming my woodwork is well done before the base clear.
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Do you roll out toilet roll paper on the wing to soak up excessive resin or polyurethane? That's what we used to do on glider wings, after sealing the surface with dope!
Cheers Neville
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I have always rolled the toilet paper over the resin on the surface, after screping most of it back into the cup with a credit card ( wife's work best!) - but always roll it so the paper stays on the roll. After a pass down the surface, unroll the resin soaked layer to expose fresh TP for the next pass. Soaks up the excess and spreads it evenly at the same time.