Originally Posted by oldred
Whatever you grind with BE CAREFUL! It is very easy to warp sheetmetal with a grinder and more than one person has managed to do an excellent job of keeping the warpage down while welding only to warp the dickens out of it when grinding! I personally prefer those flap discs or a flexible sanding disc, I never use a stiff grinding wheel but then that's just me so use what works best for you.
I just did this myself. Actually had a nice repair going, had tacked in the patch with no warpage, skipped around and filled in the gaps evenly, it was turning out beautiful. Go back to clean up the welds to get that super slick finish and noticed my panel is all warped. Never gave a thought to the fact I was overheating the metal by grinding too much.