I'm very new to woodworking and am mostly learning from my numerous mistakes. At home, I have two pine boards that sit atop my table and function as a tabletop and also as a work surface for all of my projects. (My apartment has limited space, so I wanted the table to do double duty.) The table top consists of two identical pieces of pine held side-by-side via clamps that clamp to the table beneath them.
At this point, the pine boards are pretty marred from stains and cuts (since my old polyurethane coating is gone by now), so I want to sand the boards, stain them, and re-polyurethane them. The sanding has been a nightmare, though. I have been using a belt sander with 40-grit on high power, because my previous sanding at higher grits left tearout, and I wanted to even everything out and start again. Anyway, after my first goofs with tearout (at a considerably higher grit than 40), I put putty into the tearout places and sanded with 40-grit paper using the belt sander. So far so good. Then I progressed to 80-grit, and huge tearout re-occurred! What can I do to successfully use a belt sander on pine without all of this tearout?