I have been been refininshing 30-year old wood (pine)trim in my home. I sanded off the dark finish to bare wood. The finish was moderately difficult to remove. I then stained with a natural colored stain. Before I could apply a polyurethane I had a white residue precipitate out of the wood. It is slightly crystalline and powdery looking, much like a carbonate mineral. I have tried several removal products, mineral spirits, and more sanding. The sanding seems most effective, however, it gums up paper very rapidly. Does anyone know what this may be?