The main reason sand blasting distorts sheetmetal is because each grain of sand acts like a tiny hammer. Each time it hits the metal it stretches it. Don't hold the blasting nozzle at right angles to the work, use it like you were hosing down the yard with a hose pipe. I only have a small blaster but I haven't had any problems using it this way.
SDS Jap, Do you really need to take all the paint off?
Post the question on the Exterior Forum and ask the guys.