the way I did my corners was to hit the area where the spot welds are with a wire wheel, then you can see to use a spot weld cutter to cut the spot welds out.
here is the one I used from Harbor Freight.
http://www.harborfreight.com/3-8-eig...ter-95343.html
it may be cheap but it is effective, it also helps to drill a small pilot hole in the center of the spot weld to keep the cutter in the right place