Just fyi, although it can be done with OK results (considering the low cost), it's difficult to get a good finish when you are painting downward onto something like the roof, hood, trunk, etc.
Obviously you could remove the hood and trunk, so that you can get the right angle, so the can is always upright, but I found it difficult when I painted just my car's roof. I wasn't expecting much anyway though, and I could probably get a smoother finish if I cared enough to wet sand and then buff it.

Maybe I'm mistaken, and that product is different (so angle can is held at doesn't matter) but that stuff I used wasn't like that.
Also, if you sand the surface prior to paint, and there is still paint left on the surface, you shouldn't need to apply a primer first. Not to my knowledge.