Another question about painting

I'm looking at spraying the bike but am likely to be using a spraycan method. Just wondering how I would go about spraying over part of the fairing which is already painted. For example if I wanted to put some decor on a partial section of the tank, how would I go about this? Can it be sprayed directly or mor than like I'd need to prep the surface somehow, but what would I need to do.

Any help please