I use a big wire CUP on my right angle grinder. mainly cleaning floors, it will work well to clean ou that roof seam. factory leaded such, but... cean it real good, treat with a rust killer like Osho, sealed with epoxy primer, ,, its what you will cover the car with after DA ing it, you will simply seal that with a good body filler. unless you want to learn how to use lead!
yes jack stands on the plywood, NOT dirt, is way to go!