If you are going to blast the whole cab, most do it first for the reasons Kevin mentioned and it also cleans for good welding. Maybe you are worried about rust till you get it all welded and primed. If so, maybe you can blast, see all the bad areas, prime the whole cab, then just remove primer where welding. Kevin can advise..........and others.
If you feel you can see all the areas that need patched without blasting first, you are good to go. If you run into something you don't see, just fix it then. It is a real heartache to have something surface rust while you are taking your time repairing.
