Be more careful?
It's easy to take too much off.... keep your grinder flatter if you're digging into the metal AND WATCH where you're grinding... you should be able to clearly see when you're grinding the "weld" and when you get into the sheet metal surrounding it. Don't grind into the surrounding metal any more than you have to in order to make it "flat".
Changing discs might help you with this. Use an aggressive grit to start.... knock the weld down - and switch to finer grit - finish with a 3M "pad" and even these come in various "grits" (they're not measured in grit - but you get the drift).