You can probably just sand away the problems and scuff the rest of the surface. Primer might be a good idea, and a necessity if you are going to a significantly different paint brand or recipe.
Even spray paints can take days to truly cure, the longer you wait, the better.
"Several thin coats" is probably the best advice of all. Most spray paints usually say something like "re-coat within one hour or wait 24 hours" and in that scenario 3 thin coats 20 minutes apart will give you a very nice result.