How to prepare previously stained trim / baseboard for painting?
Regardless of the religious factor involved in painting hardwood trim, (lololol) if you must do it, you need to buff sand the wood, 150 or 220 is fine for this step. PRIME the trim with BINs Bullseye, pigmented shellac.  After the Bins dries, very lightly sand it again with 220-400 paper or 4/0 steel wool. It will be smooth as glass. The shellac will seal the grain and block any bleeding into your new color.  Now go ahead and paint away!