ORIGINAL: combatpigg
I tried WBPU on cloth but didn't think the sanding of the seam edges went very well, it got fuzzy. Epoxy isn't any picnic either, it seems to take forever before it is set up enough to sand without being gummy. I like polyester because it gives the hardest shell, but can't touch foam with it. Foam will soak up epoxy resin like a sponge on the first filler coat. Laying up cloth without filling the pores of the foam first is asking for incomplete adhesion. Is there something that you can lightly spray onto the foam [like maybe WBPU] to seal the foam without gaining a lot of weight?
I had an old foam wing that I tried some testing on -- treated each panel differently. I took some lightweight DAP spackle and thinned it down with water to a thick cream consistancy, then painted the panel with a foam brush. It went on pretty well and filled in the porous portions of the foam. I sanded that down (it needs to set up long enough so its not gummy) and added enough coats so the foam was not porous. The filler in the lightweight spackle may be microballoons since it floated if I added too much water. Once it seemed to be fairly smooth, I came back with Minwax WBPU. Found out later that Minwax is not fuel proof. I've also tried .5 oz glass and WBPU -- it added a lot of stiffness, but was a little heavy. I didn't try it, but mixing lightweight spackle into WBPU may improve filling and save a lot of weight.
I've also been using DEFT lacquer sanding sealer on wood -- it's light, seals the pores and leaves a hard finish, but attacks foam. If I could figure out how to lay down .5 oz cloth and seal the foam, then come back with the sanding sealer, I think it would fill the weave and also be pretty hard.
I think BEHR WBPU is supposed to be fairly fuelproof and may work better.