I've had good results with Timbermate to grain fill mahogany under solid and transparent nitro finishes - smooth as glass. The mahogany is nowhere near as open in grain structure as ash though. I have an ash strat kit in build that needs finishing so would like to hear what people suggest to do to get a nice flat finish with transparent or solid colour without running into compatibility issues.

With ash, I've heard of folks pre-treating the body with shellac > grain filling with timbermate > sanding > shellac again > nitro sealer coats > nitro colour > Nitro clear. I think the shellac is thought to stabilise the timbermate shrinkage. I've not tried this though. I was thinking about a clear epoxy or CA grain fill but like KC, don't know what products people have had success with in Australia.