grain fillers... hifi

In the past I have used timbermate and various sanding sealers with reasonable sucess.
I work a lot with plywood, the luan ply is something I have used in the past.
I have considered that going up grade in the ply material could save me quite some greif (the burch ply for example is much closer grained) But thts not the point.

Since going across to nitro laquer.. Ive found the grain filling is far more critical if one it trying for a piano finish (or similar).
The luan is quite open grained and takes some filling, re sanding after filling with timber mate seems to pull the filler out of the pores and/or exposes more unfilled voids.

the problem is air comes up thru the finish leaving bubles in the finish, sometimes this persists coat after coat with nitro, particularly if you are heavy handed with the spray gun. There are methods to combat this but nothing I've found to be totaly effective.

This problem doesn't occur with poly as the previous coat isn't resolved by the solvent in the following coat.

anyway cutting to the chase.

I am considering other grain fillers.
What is there on the market redily available?
I'm lookig for something that has serious ability to bung up open grain.
The particulate matter in the filler will need to be pretty fine.
Probably some sort of resin base rather than a simple paste.

I'm not interested in french polish or burnishing styles but a rub it in sand it off then lay on some spray coats.

aly ideas... comments.

cheers