I think if the paint is cracking it's in the first stages of failing. It's hard to say without seeing it. It may be that whoever painted it applied too much and didn't allow enough drying time between coats. The doors probably should be stripped to be sure. If you have them sandblasted be sure you get someone that knows what they are doing. It would be easy to burn a hole through an aluminum panel. I would probably take the door apart and do one section at a time with Kleen Strip paint and varnish remover. Use in temperatures above 70 degrees. Brush it on and keep the dry spots wet with remover for about 15 to 20 minutes and rinse off with a power washer. When painting back, epoxy primer is the wrong product. Use a primer such as Dupont Variprime or if you are looking for something cheaper use zinc cromate primer. You can then topcoat with any automotive paint or oil based enamel. If you wish to go back with latex primer I would put a coat of Preprite primer from Sherwin Williams over the Variprime first.
Another option would be to contact a overhead door company and see if you can just purchase new panels.