Francesca has it right, there really isn't any need to blast or strip the paint off an aluminum sided trailer. Blasting should be totally out of the picture. No matter what media you blast with, there will be panel deforming.
If there are spots that the paint is showing bare aluminum, they should be primed. In fact, I primed all of the body of mine as I wanted good adhesion.
Here's the drill: Scuff sand siding, wash with a phosphoric acid based prepaint wash, rinse, mask, shoot with primer, shoot with topcoat, let cure, mask from areas needing accent colors, shoot accent colors.
I used Sherwin-Williams DTM industrial primer and standard latex siding paint for the body. Accent colors were from rattle cans.
The done deal: