Currently I'm liking the Mar-Hyde (now 3M) Ultimate 2K primer. It fills well and sands like butter...not too expensive either. Seems to hold out and not shrink hardly at all. I did recently use some of the Clausen All-U-Need, which is pretty much spray bondo, and it really fills well but sands like garbage. It's fine for rough work, and I would never try to use it for a final finish product. Good for a first round of priming and block with 80 or 120 and that's about it. The Ultimate 2K will fill 80 grit scratches with 2 or 3 coats max.