Flap Disc

Right, what I was trying to say was if you're trying to knock down welds quickly without generating excessive heat then use a 40-60 grit. 80 grit will work but but tend take longer, thereby generating more heat, and "polish" the weld instead of knocking it down level quickly. If you have nice, flat welds to begin with you can get away with finer grits.

Personally, I find the finer grit flap discs don't last as long. Once they're worn down a just a little ways they kinda peter out. They will last longer than a single layer grinding disc though.