Since you've already used dope-based sanding sealer, I would recommend using clear dope and adding talcum powder until it's fairly thick, then thin it to brushing consistency. Sanding sealer isn't really designed to fill wood grain. It's what it says, a sealer applied after sanding to fill the very fine scratches that are left.

Actually, there's no reason you can't add the talc to the sanding sealer to make it fill the grain better. Just thin it down until it brushes out smoothly. Allow the fill coat to dry well, sand it almost all away, then apply another coat. The idea here is to fill the grain and not have any more filler on the wood than is necessary.