The sanding sealer I have comes from Cornwall model boats and is labelled JP.... its xylene based so I use Xylene as a thinner.... this stuff really smells bad and the smell lingers so use outside if possible.
I normally thin mine down to with about 30% extra xylene for the first coat, this makes it really soak into the wood. then once dry another coat of undiluted SS and then sand, applying another coat if needed.