I'm about to refinish some oak hardwood strip flooring that have gaps that have opened up between the flooring strips. In many articles I've read, no one seems to mention using a troweled on wood filler to fill these gaps. So, my questions are 1. should I use a filler, or just sand and polyurethane and leave the gaps as-is? (The concern I have with filling is about what will happen when any expansion occurs in the summer. The room for expansion is now filled with filler.) If it's recommended that I use the filler, two other questions... 1. At what point is the ideal time to apply the filler (i.e. after initial passes with sander, apply filler, resand/screen floor with 120 grit, then poly?) 2. Any recommendations on products to use/not to use? Thanks!