Can I use a fine sand paper to remove old wall paper paste on my walls?
Hmmm... I wouldn't! But Chances are you already have... I would use a Hair Dryer, to kindof temper the materials to uncohesively pull of the paper. Kinda like you would tape.
At Lowe's and Home Depot now they have a paint that you paint directly over the wallpaper. I haven't used it, but I saw someone on HGTV use it, and they loved it.
Yes you can, but if it the paste is hardened, then use a rougher sand paper.