"jtomasik". I respect the fact that you took it upon yourself to do a search, that's exactly what the "search" feature is for. I've read the entire thread and some of the advice was less than stellar. Mike "Deadbodyman" is correct about the method for applying adhesion promoter...however, if I read your post correctly, your asking if there is anything out there today that will give you better adhesion on plastic or rubber bumpers than what you have on it now in the event of getting rock chips or minor scuffs.
If that's your question, the answer is yes. This thread was started in 2009 and the reason I mentioned that the advice was less than stellar is because products where available then as well that would outperform some of the finishes that exist today. The person who started the thread was using aerosol spray cans to paint/repair his bumper. Other than adhesion promoter and even adhesion promoter, when applied with a spray gun goes on much more easily, is atomized better to give more even coats and well, in my opinion, aerosols for the most part, should be used for spraying model car.
What can be done on these types of bumpers is to clean the surface with a quality Wax and Grease remover, repair any chips or scuffs with sandpaper, I would use a 320 grit paper, feathering the effected area back so that the chip or scuff seems to be non existent when you feel it. If your down to the raw plastic, adhesion promoter should be applied, two medium wet coats and allow them to flash, flash means that it is not wet anymore to the touch. Depending on brand, read the instructions, you can now apply a quality Epoxy primer. I prefer SPI Epoxy for several reasons, it lays down nice and smooth, has respectable build and incredible sanding qualities for an Epoxy. Now, an Epoxy primer is exactly what it sound like, it's basically a glue that acts like a primer and remains flexible (very important on rubber or plastic parts...in case you get into that little mishap). I would recommend 2 coats of Epoxy, let it cure, usually 24 to 48 hours and sand the Epoxy with either 400 dry or 600 wet paper. The Epoxy primer will act as cushion between your plastic/rubber and your top coat/paint. Clean the surface with a quality Wax and Grease remover, make sure that it has completely dried and apply your base coat and clear. When your applying your clear coat, you could add "flex agent" to your clear so that if the bumper, if disturbed, the clear won't crack as easily.
By doing it this way, you will have a better, more flexible paint on the surface of your bumper. If you use rattle cans, for the most part, you will not get the results it sounds like your looking for.
I hope this helps and, just so you know, you did the right thing by doing a search before posting...if you would have wanted to start a new thread, that would have been Okay as well. If you need any more information, please feel free to ask.