Sanding sealers contain a soap-like lubricant that makes the sanding of the first coat easier and faster, but they weaken the bond of the poly finish. So you?re better off not using a sanding sealer under poly. Also polyurethane bonds especially poorly to sanding sealers, so most manufacturers of polyurethane discourage their use. Furthermore, polyurethane sands easily, so there isn?t any need for a sanding sealer.
Nevertheless, the existence of primers and sanding sealers (and the recent addition of ?SealCoat,? a dewaxed shellac from Zinsser, marketed for use as a ?sealer? under polyurethane) have created a belief among some that something has to be put under the polyurethane to make it bond better, but in fact this is just not true.
I never use anything under poly when I use it other than stain, that is if the customer wants a particular color.
Sorry but you might have to sand it back down to the bare wood and start all over again.