walnut shells work awesome. The big thing with shells is that it'll remove the paint FAST, but won't attack the aluminum underneath.
Depends on the thickness of aluminum and what kind of final finish you want.
I've seen them blasted with sand before. Fine Arkansas river type sand, real dusty, really fine stuff. That was a .125" thick hull material. Was finished out with automotive style paint and came out awesome. Actually I got the sand from the river, ran it all through a sieve to get the shells and sticks out and ran it though the blaster. Took a while with one of those 5 gallon pressure blaster kits that I picked up at Harbor Freight, but it worked excellent at 90 psi.
For a boat with thin aluminum, stick with walnut shells or soda. Glass is a bit too aggressive for thin stuff. Think soda can. I've blasted soda cans with baking soda before and it removes the anodizing, bare aluminum, and does not even begin to pit or dent the can.
There are different "grits" of blasting media, even among the same media. Al oxide can be from 12 grit to about 220, for example.
A good blaster, someone who does a lot, can strip a car body in about 10 minutes. An aluminum boat, about 15 minutes at the most if he knows what he's doing. Takes longer to set everything up and get suited up. Some guys just don't want to mess with it but instead of saying "I don't want to do it", they'll jack the price way up in hopes of running you off; but if he doesn't succeed in that, he'll make some money which is what he's in business to do.
I know a guy who does (or did?) some soda blasting but AFAIK, I think he's about done with it (retiring). He's done some soda blasting on different classic car bodies over the years, excellent work.
