I use the cheaper $12 blast gun with a gravity feed hopper that holds 2 bags of sand.
Cruizin, if you're only doing the bottom I wouldn't blast at all unless the car is completely apart AND I would NOT blast inside the garage, unless it's EMPTY. Sand will get to the farthest corner and get into tvs, radios, furnaces, intakes etc.
Under the car, if not apart, will get into every crack and crevice in the body,frame, k-frame, suspension, steering, shocks, etc. You get my drift?
I blasted just the engine compartment about '84 and still find sand here and there.
For underneath, if undercoated, use a propane torch and scraper if the interior is out. If still in, spray it with Tar and bug remover (somebody suggested here) to soften, then scrape. Then use a wire wheel or grinder before using Rust-Mort to kill any rust. SEM product
Rob