I think the rattle can could come off easily. Was there any sanding before the rattle bomb went on? I'd never blast with a project like this. I'd grease remove with Gunk, then trisodium phosphate. Then jitterbug with Scotchbrite and Scotchbrite wheel on a drill motor for the nooks. Either this takes off the black, or if not, then it's sticking well and shouldn't cause a problem. Then wash the engine compartment with tsp and a Scotchbrite again, or use something called Ting, which is available at autobody paint supply shops. With the car prepped like this, you should be able to find someone to put the paint on.