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Building Tips and technical articles. => Paint and finishing => Topic started by: Charles Hofacker on September 27, 2020, 09:09:01 AM
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Do y'all put tissue over bare balsa sheet? or fill the wood grain with primer/filler? I tried putting tissue over bare balsa sheet on a profile fuselage with clear dope then filling and sanding and have several problems: 1) tissue over laps: every where the tissue overlaps it leaves two ridges that print through even after much sanding. and 2) wrinkles in the tissue: some areas of the tissue were wrinkled before doping the tissue down. Now those wrinkles seem to be permanent - I can't get them out even after much sanding. If you you do use tissue over bare balsa sheet, what is your technique? What kind of tissue do you use? Where do you get it? It seems that silkspan is no longer available. Poly Span and other polyester(?) tissues are very difficult to get to go around compound curves and seem to be a major pain in ascii to sand...
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Yes, I cover bare balsa with silkspan. I get medium weight silkspan from a place called Easy Built Models. It is the real stuff, just like we used to get years ago.
Here is the way I do it, but there are other methods that will work: I apply clear dope, maybe two coats, to the bare wood. Sand lightly. Dampen the silkspan and apply dope thru the wet silkspan. After this dries, I will apply two more coats of clear, then sand.
As far as the ridges go, it is better to tear the silkspan, leaving a somewhat ragged edge. After several coats of dope and sanding, this edge will disappear. Do the same with the overlapping piece and you will hide that edge pretty well. A cut edge, as you discovered, is harder to hide.
I see no advantage to using polyspan over bare wood.
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I've used both silkspan and "domestic" (cheap) tissue. For this application, even tissue from shoeboxes seems to work. On flat or mildly contoured surfaces I only tack it around the edges, and then shrink it with water (if it's tissue) or put it on damp (if it's silkspan). I've found that silkspan has to go on damp (although I think this may vary by brand -- I've only ever used Sig). Tissue can go on damp or dry. Once it's dry I go over it with dope, and it sticks right down.
Don't think to do this with Esaki tissue -- the stuff is so nice and tightly made that dope won't soak through, and you'll never get complete adhesion with the underlying dope. You've been warned.
For really contoured surfaces like wingtips or cowls, I'll use strips (torn, as already mentioned) put on damp and held down with dope. You want to start at one end and work it progressively, keeping wrinkles out as you go.
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If you do use Polyspan over bare wood such as on a profile fuselage, it will make it incredibly strong. That is the advantage over silkspan.
Mike
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If you do use Polyspan over bare wood such as on a profile fuselage, it will make it incredibly strong. That is the advantage over silkspan.
Mike
Silk will make it even stronger and the silk will lay down easier.
Both those materials tear easily, silk won't.
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Tissue covering over wood increases torsional strength considerably. This permits the use of thinner wood and results in stiff construction with less weight. Silkspan or paper tissue is a bit more difficult to manage over sheet balsa, so I like to use carbon "veil". It dopes down easily, and the seams disappear completely after a few coats of dope.