Good question! I have never experienced a problem with this although all the surfaces I've covered using this method have been sheeted with no exposed foam. I can tell you it has never wicked through the sheeting and attacked the foam core. You are not adding allot of acetone to the mix. I find that for one pump of resin/hardener a 1/2 oz of acetone is enough to thin it. West systems epoxy is much thinner than you typical finishing resin like z-poxy which I had used for years (good finish but shrinks too much for my liking). For glassing wings I find it best when you get the resin thinned to a milk consistency, then just pour it on the surface and spread with a playing card/credit card. If you are concerned with acetone attacking the foam just use denatured alcohol to thin....
Todd