subject comes up often here so if I don't answer it completly you can search on it.
If the bumper is already primered, then all you need to do is clean it well (wax/silicone remover) and scuff. Then you can shoot. You don't need the adhesion promoter as that's meant for unprimered plastic (in this case).
I would scuff with something like 600-800 grit wet or dry but go easy since you don't know how much primer is there.
Topcoats on Bumpers may or may not needed (see P-sheet for the product your using) a Flex-additive but Clear Coats often do.
If your having a peeling issue it's probaly due to the surface not being prepared properly prior to painting. Either not cleaned well or not abraded correctly.
You'll need to keep sanding over the area until you get back to where it's good at. So if your sanding and it keeps peeling keep sanding the entire area until there is no more peeling. Otherwise whatever you lay on top will not last for long. See here for a more technical explantion of causes and repair.
Last edited by Roch_Greg; 07-04-2006 at 10:29 PM.
Thoughts and comments expressed by me are mine based on my own experience and research and shared here freely. I am not a professional nor make any claim to be as such