Flap Disc

First of all, welcome to the wacky world of whacking metal!
Next flap discs come in different grits. The coarser the grit the more aggressive. For general clean-up 60 grit is good, 40 will take steel down quickly, and 80 is good for cleaning up butt welds in sheet metal. Other than that the coarse ones are great for shaping wood!
Harbor freight has good ones at half the cost of the ones at HD.
Andrew