I am trying to refinish an exterior wood door. It had multiple layers of paint, most of which have been removed. I would like to stain it to match the rest of the wood in my house. Problem is, there's still paint deep within the grain and in pockets where the wood has been dented. Is there something I can use to fill the gaps and give me a completely clean surface to paint. If I continue to sand, I'm afraid I'll take off the veneer completely.

Thanks!