Chuck,
I've been using a shrinking disc quite a bit on my fenders and other crowned panels. A shrinking disc works by heating (locally) the high spots that it contacts. When the heated spot is quenched, it shrinks. When you repeat the procedure, it works on the "next highest" spot. In theory, it seems like it would be effective on a flat panel, especially on small raised spots. I think that for a large flat area you would have trouble controlling the shrinkage. The shrinking disc probably isn't the solution.
I agree with MPandC on the stud welder. You may want to spray the area with guide coat and run a block sander over it to help visualise the highest spots. Work on them first, then progressively until it's flat. Work slowly.
Dave Evans
