Auto body repair & detailing: roof skin removal, spot welds, air chisel

Question
hi, i have a 1983 gmc truck that i want to remove the roof skin and replace, because the sunroof that was improperly installed. i am not sure how to seperate the lower and top halves.

Answer
Putting on a used skin is a real bear, going on and coming off. Remove the front windscreen, center punch (Important!!) and drill out all the spotwelds located along the pinchweld. Use  a 1/8 in. bit first, then a 5/16. Open the doors, drill out all the spot welds along the drip rails. At the seam at the back of the cab, use a cut off tool and wheel, and cut the skin just above the seam- don't cut too deeply, try not to damage the part of the cab you need to save. The more careful you are, the less work you have to do. Once you liberate the skin from the cab- it will take some pounding with an air chisel or equivalent- grind the pinchweld seam you exposed at the back of the cab flush. Don't worry about the extra layer of sheet metal left from the old roof skin, just grind it flat until there is no bevel on it from the old skin. Now, do the same to the spot welds on the donor roof skin, but along the back seam do this instead of using the cutoff wheel- use a sawzall to cut underneath the seam. Leave at least a half inch of extra material so as to not damage the roof skin. It's going to take a lot of cutting and trimming to get the skin off with the extra bit of cab, but it can be done with some patience. Once you get the skin and cab back liberated, you need to drill out the spot welds holding the rear cab sheet metal to the roof skin. They are hidden inside on a pinch weld you can't access until you chop the roof to pieces, you see. Sound like fun? It's not. Hammer and dolly out the pinchwelds to remove any distortion from the removal process, and grind off any rust and paint. I prefer to use structural adhesive as opposed to welding. You should plug weld the skin on at 4 corners, and use glue to hold down the rest. This reduces welding and the chance of a fire, or other weld spatter damage to the interior or glass. You can find auto body adhesives at any store that sells professional automotive refinish products. Apply the glue fairly liberally on the inside of the donor skin, along the pinchwelds you drilled out. If you insist on welding without gluing, cut the old roof 3/4 inch tall at the back cab seam, and cut the donor skin just above the seam, but leave enough of a gap over the pinchweld seam to be able to do welds without ruining the natural seam line at the back of the cab. Overlap the new skin onto the old, weld the seam by bouncing around and doing only half inch long welds. Plug weld the holes through the drilled out spot weld holes. I like to use sheet metal screws to hold skins in place after I fit them, when I weld or glue them, it holds them in place. Bill