I recently finished removing all the underseal that had been painted all over the inside of my Clubby. As others have said, heat gun and scraper brought it off pretty easily and I used a cloth soaked in white spirit to wipe over afterwards and you'd now never know there was any underseal in there. Just got the plethora of rust that was hidden underneath to deal with now

the funny thing about sand blasting is you can buy the gun at supercheap auto as well as the sand or whatever blast media you want, so whats stopping you doing it in your backyard.
ive seen people use these guns and use an portable enclosed carport to blast cars in them. the enclosed carport stops the dust going ever where.