Whelp I went ahead and added the grain filler. It?s like nothing I?ve ever used before (which doesn?t say much!). Basically it?s a thick gooey paste the texture of a heavy machine grease, which goes on bluish white and turns clear. Suspiciously like an ultra-thickened polyurethane finish, I wonder if it?s a similar chemistry?
It does shrink somewhat, as evidenced by crackling in the deeper areas filled, like >1/4? diameter nail holes that I hadn?t totally filled to the brim with epoxy in an earlier step. This AquaCoat filler probably shrunk by 10-20% in volume as it hardened in those areas and the crazing resulted. Oh well...I really don?t think anyone will notice!
The call to the manufacturer didn?t go through (I think it?s my phone, hopefully) but I found a lot of online reviews suggesting it will work over polyurethane. The manufactures instructions don?t warn against that, either.
Fingers crossed but I think it?ll work out and be ready by Saturday!