I would recommend you stop painting your body with spray cans. Spray can paint is uncatalyzed, ie no catalyst/hardener mixed in there. That's what gives epoxy primer its real bite and hardness.

Now if you want to do your car in real epoxy primer, that would be the way to go. The p sheets for the type of primer used will tell you what grits the manufacturer suggests but typically for epoxy primer I sand down bare metal with 180.

You can buy an electric sander instead of an air powered one. They work pretty good as well. A local auto body supply shop will be able to sell you a box of 180, round sheets with the velcro backing for your sander.