emmm... directly onto the paint... im not sure about it yar, compound is a bit harsh type of thing and im not sure what it will do to not-so-sticky paint... offcourse the spray paint is very different to the car's body paint (for which the compound is actually made)
why dont u do another project, take the panels out, respray if they have scratches or simply give them a clear coat!!! this opening and fitting back thing is a mess but the result makes one proud
and yes, avatars become ones identity here ... thats why i havent changed mine for like 4 years now