Need some help from a rust guru!

You are basically correct, and I have tried it all over the decades. The sand, you never get it out! I tried chemically striping a hood way back in early 90's and it worked OK but I had to flood the insides of the frame with primer and paint. Not worth it all.
Back in the 90's, I had a guy with a commercial blaster that had done it for years We raised the cars up with a boon truck and he cleaned the bottom sides and the rails. One day he saw I had a 69 RR hood in the back of the truck. He wanted to blast it and I said H--L NO!! You will ruin it. Well he blasted it and never worped it at, all BUT he was probably one guy in a million that could have done it!! LOL
MY method is IF I have a survivor car (few an far between any more) with good OEM primer and it is GOOD, I will run a palm sander over it with something like 180-220 then epoxy primer, otherwise I DA it with 220 being careful of leaving sand scratches.
I have a steel horse trailer that needs some blasting and I would like to try that new Dustless deal or whatever its called, with it if I could find anyone doing it that is NOT in Houston!!!!