Question About Sanding

I just glanced at your post, so I hope my experience helps with what you are going to be doing. I’m on the road this morning.
Number one: Sand blasting takes a lot of air if the job is of any size. A small compressor won’t cut it. You’ll just get frustrated. Maybe if you hook two compressor together. I've done this.
Number two: If you can’t get all the rust off. There are options. I have had good luck with Rustoleum's “Rusty Metal Primer”. This is a red primer available at Home Depot. This is what it is made for. It comes in spray and can types.
Get the loose stuff rust off and use this. I won’t say the rust will NEVER come back, but probably not in our lifetime.
You will probably want to overcoat it with a lacquer based automotive sandable primer before top costing it.

I know people have used types of rust reformers. I tried one and didn’t like it. Plus they cost a lot more.
Number three: You know that going from 80 to 220 grit is a big jump and will make you sand forever with the 220 to get the 80 grit scratches out of something. Sanding should be done in steps.
Number four: Are you using a sanding block? I have one-inch think pieces of rubber in various sizes. That is what you need to save your hand and get a flat surface.

“What the eye doesn’t see the bodyshop gets away with.”
Keep repeating this to yourself.