[TimE]

What causes the cab to rust like that is the seam sealer around the roof dries out, cracks and lets moisture leak in between the seams in the roof. I agree with the other guys that it is probably pretty crusty under there.

If you're like me and have more time than $$, you could drill out the spot welds on your roof, take the top off, clean it, seal it, repair the rust issues and put it back together. It would be time consuming but you would know then that there would be no future issues with it.

My cab isn't nearly that bad but I may take it off so I can clean and seal it so it doesn't come back on me. I only want to do my truck once!