Blake , sanding metal in one spot too long can cause warping of the metal panels, just like not alternating welds when joing panels. The area gets heated up by the friction of the sand hitting it. Most commercial blasters(ones who do it for a living) use walnut shells or other medium because it is easier on the metal panels not heating up the area but still removes the majority of the paint rust or filler. Just take it slow and dont spend too much time in one area of the panel. Move the blaster around alternating positions on the metal as not to heat it up. Dont get in a hurry and youll be fine.
