just posting to say yeah, do the sandblasting. cant help you on the other issues, of airtightness etc.
my frame was sandblasted, quality red oxide primer, then just black frame paint. ten years ago still no rust. i have C channel frame (from a 79). good luck with the toy. i would maybe use a powder coating company to sand blast if you have to but i would NOT powder coat an off road vehicle due to the chipping issue because you are going to throw alot of rocks, and maybe drag your frame over them too. not like that wouldnt remove the paint either, but still. easier to fix paint than pc.