Billski, IIRC, in a recent post you mentioned that you'd used straight epoxy to seal the foam in prep for laminating, rather than using epoxy/micro slurry, and that you'd seen little/no weight increase. Do you suppose there's any reason to be concerned that straight epoxy might not give the same foam/laminate bond strength immediately and/or long term? I don't have any reason to doubt the ability of straight epoxy to do the job, but I also have very limited tech knowledge of these materials.
Hmm. Starting from the beginning, we need to fill all of the broken cells in the foam surface or the resin we add to the cloth will end up filling it while displacing the air bubbles into the glass, which reduces the interlaminar strengths. So, we could fill with neat epoxy, but using a 50/50 mix of resin and microballoons and squeegee off all of the excess is lighter and works just as well, so that is what we do with open wet layups. The slurry is weaker than neat epoxy, but it only needs to be stronger than the foam to work at attaching foam to the glass-epoxy skins.
When I went to vacuum bagging, my experimental parts with slurry on the foam had the micro migrate into the glass cloth when the vacuum was applied, and that is BAD for interlaminar strengths. So I wet my foam with neat epoxy and let it be heavier than if microslurry was used. I believe that I conveyed that my vacuum bagged parts were plenty light already, not that they couldn't be lighter with micro. My apologies if I mislead on this.
If you were really after every ounce, and zealously seek less dense surface under your fiber-resin layer, you can do what is called hardshelling. You seal the foam with a somewhat wet micro-resin, squeegeeing excess from the part, let it cure, then sand it with 200 grit just before vacuum bagging on the facing. This is usually done where the part has a lot of shape or curvature to it. You laminate the first facing against the mold and then the foam, bag it and let it cure. Then you hard shell the foam and apply the other facing. This scheme only works for one side, but if you had the foam in net shape (curved surfaces and edges) I suppose you could hard shell both sides.