Flap Disc

Flap discs have layers of paper abrasive so they last longer than a single layer sanding disc. They can generate a lot of heat fast. An aggressive disc will cut faster and generate less heat because you're not "polishing" the weld you're trying to grind.

If you're not real experienced and making lumpy welds I suggest you take the edge of a cutoff wheel on a die grinder and grind/cut the "mountains" down first then finish them off with the flap or other disc.