All the windows and wood trim in my house are stained and polyurethaned. Every couple of years, I see areas around the window where the poly finish is flaking and peeling. I usually sand, wipe it down then apply a fresh coat of poly. But then, a couple of years later it starts to flake again. I am thinking that I am not doing enough prep to make the poly adhere well. This time I thought I would clean with crud cutter, sand, then wipe down with something - but what? Denatured alcohol? Mineral spirits? I am not sure of the best way to do it so it adheres better. I don't think its a moisture problem, there is no mold or dampness. Suggestions?