Painting bumper filler panel

So the kiddo bought some spray bombs at NAPA, custom mixed to the paint code...and surprise, they are not a good match...shop is redoing them (hopefully we get new ones, not the ones we took back) to try again.....

That said, the paint the shop says has flex agent in it so its good for plastic etc.....

But it got me thinking....what about priming? What kind of primer will work on the fillers and be compatible with the NAPA spray bombs?
Author: shadango