body filler application

Hellooooo FABO,
I have done a lot of body work on my car. It looks pretty good, but its my car. I now have a side deal that is going to require filler on someone elses car, so.......I am seeing different methods out there. Some say the paint must be removed, some say it just needs scuffing, and some are saying it needs to be bare metal then primed before filler applied.

What say you? its a rear quarter on a late model non mopar so Ill spare you the details on that, but only mention because it is a rust free situation with fairly thin metal.

Thanks fabo!!