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Building Tips and technical articles. => Paint and finishing => Topic started by: Geoff Goodworth on April 20, 2014, 05:34:04 PM
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Has anybody used water-based polyurethane and polyspan covering without attaching the covering with dope?
I'm keen to try this as I have good reasons for using both materials but I'm wondering about attaching the covering.
Should I use Balsa-loc or thinned PVA wood glue and iron on the covering before sealing and finishing with the polyurethane?
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I don't know about Polyspan, but this works great with silkspan.
I do a couple of coats of Polycrylic on the bare wood, then I wet the silkspan, wring it out so it's just damp, then stick it down with Polycrylic. It has a tendency to want to expand in the Polycrylic, but starting with it damp makes the job easier.
I have no clue how Polyspan would work.
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Tim's right, you can get it done using Polycrylic. However, Polycrylic is water bast. Not a good thing to do apply a water base chemical to bare wood.
I gave the wood a couple coats of wood filler, lacquer based. dries fast sands easily. Home Depot.
All this silk work was done using lacquer based wood sealer and Polycrylic.
Charles
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I have used water based polyurethane on bare wood with no problems, even the flat surfaces like stab/elevator, flaps and rudders. Yes if you put it on one side first they will start to curve a bit, but I put it on like doping wings. It seals the wood and for me makes a good base for dope after sanding all the fuzzies down. Then I go with a few coats of dope to have something to adhere the poly span(now Poly-T if you order from Stunt Hanger Hobby) with. I put it down dry pulling as many wrinkles out as I can. Once all surfaces are covered I carefully go over them with the heat gun. I have also discovered on fuselages and flat surfaces that a iron will take out 99% of wrinkles that have formed from trying to get the Poly-Span to lay down while doping it down. Use the iron after dope is dry. By the way the polyurethane was a mistake at the time. I thought I had opened a can of poly-acrylic clear. It still worked for my application though.
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I put a very thin first coat on, to avoid warping. I have had wood warp with Polycrylic, but that was some 1/16" center-section sheeting that I had really jammed in place rather than sanding it a bit more (and risking a gap). I've never had a problem with anything thicker than 1/16".
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I decided to reply to this instead of a new topic...
Is the stuff that comes in a SIG kit box silkspan? I tried brushing on minwax polycrilic (which i use exclusively now as my clear base) but i could not for the life of me get it to stick down the silkspan... I brushed on the polycrillic, stretched on the damp silkspan then brushed around the edges. When it dried, the silk tightened up a little and pulled right off the edges. I started over with another method and it worked fine. What did i do wrong?
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Dane:
Yes, the stuff that comes in Sig kits is silkspan.
I have only ever used Polycrylic to stick silkspan onto solid wood. If your trouble is coming when you try to put silkspan on an open-bay structure, I suspect that the silkspan is drying faster than the Polycrylic, basically pulling things apart before the Polycrylic has had time to develop any strength.
I don't know what, if anything, the answer to the problem may be.
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Ok. That makes sense. I'll just do it the old way. It's too easy of a job to complicate it with experimentation.
What you say Tim, is exactly what it felt like.
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Ok, so I stuck the silk span on and then used polycrillic to seal the silk. I'm gonna use rustoleum to paint.
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I'm building a Blackhawk Fokker right now using Polyspan and Polycrylic (Minwax). I've had luck so far using a glue stick to tack it down. Be sure to get the grain in the right direction. I also brushed a coat of PC on before covering to seal the wood. Brush on the PC around the perimeter and ribs and allow to dry, then use the heat gun on low.PC will not shrink the polyspan as you know. I have been shrinking it fully after the first coat.
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Douglas,
Did you apply the glue stick around the perimeter? Or just on the tough spots? And the silk span was damp as normal?