Nate,
If you're going to leave the old screws in to keep paint from getting in the holes, just make sure to back them out a bit. If you do that so that they're somewhat loose in the hole but still blocking the paint from getting in, you should be fine, I would think.
Also, if you're painting it in pieces, I'd go with soda blasting and painting on top of the cleaned case after that, but it sounds like you're just going to be painting it with it all together. In that case, prep the surface and then you can use the VHT primer under the VHT paint.
Ron