What grit are you using on your block?
I'd start off heavy on the mud, then cut it to shape with 40 grit. Going super coarse gives a good cut, which stops the block riding along on the surface, creating waves. Once you have the shape sorted and can't feel any weirdness, move up the grits until you're where you need to be (120?).
If you're already doing this, ignore me.
Palm Springs CA
Flying a 1987 O-360 RV-4, rebuilding a 1992 O-360 CS RV-6