I thought I'd share this since it worked out so well. I have a 1950's ranch-style house and the heat registers have have been painted with latex paint probably 10 times. They looked nasty with globs of latex paint on the ribs, etc.

I have a sandblaster, but you can't really sandblast layers of latex paint because the stuff is like rubber. If you use the kind of pressure you need to take off latex paint, you'll warp the metal. I hate to use the newer vents you get at Lowe's because they're practically made of tin foil instead of the thicker gauge metal they used in "the good old days". Plus, why buy something new when the used one is available and better than new anyway?

So I got an old enamel stockpot, and my turkey fryer burner and put the old registers into the pot filled with water. I added two cans of Crysta Drano and boiled them for about 15 minutes. By that time all of the latex paint was stripped off and just a few spots remained along with some rust. I cleaned that up in my sandblast cabinet (a wire wheel would have worked too, but sandblasting is better) and painted it with Rustoleum white and they came out like new! Just cost me the price of two cans of crystal Drano to do about 20 of them.

Look up bead blasting in your area. They will use an air compressor to blow silica or glass beads at high pressure to remove the old paint.

Then paint them with spray primer and your finish color and Bob's you're Uncle!!!!!!!!

the op is correct that blasting with beads, sand or even coal slag literally just bounces off. I have a cheap free standing blast cabinet and tried to clean something that had enamel paint and all it did was just dull it.
Many professional shops will turn down parts that may have lead paint due to the contamination and expense of getting rid of the media with lead residue.