HELP with bodywork

i dont understand what is so bad about the rattle cans....from what you guys are saying, you make it sound as though im going to totally ruin everything...im starting to get worried now....will the rattle cans work and hold up, or will it ruin the paint and not have it stick? i need to know because that will change everything for me.....like i said i cant spray the primer, its just not an option