I've searched and read most of the plastic bumper prep threads, but I have an odd twist:

I recently bought this car and the young gal who owned it had been in a minor front end collision. Whoever did her body work was obviously on a budget, and one of the cut corners was that the new plastic bumper pieces were never painted. They were installed on the car as-is and the car was driven for a while (don't know exactly - I'd guess at least a year). The main bumper piece has a decal indicating it is primed and ready to paint.

My question then, is... should I treat this like a brand new plastic bumper part and trust that the primer is adequate? Or should I scuff and respray with adhesion promoter just to be safe?

I actually have 3 plastic pieces potentially requiring differing preps. The main bumper piece appears smooth and will clean up nicely. One lower skirt (a separate piece) has some rash which I will sand out, apply AP, then apply 2K high build, and sand prior to painting.

The other lower skirt was missing entirely, so I picked up a new, "primed" aftermarket skirt. My understanding is that I can clean/scuff with a gray Scotch-Brite pad and paint without using additional AP. Correct me if I'm wrong.

Thanks for any and all advice. I've been spending countless hours on here learning lately!

AM.

If all these parts truly do have a coat of primer and your not hitting raw plastic sanding them you sould be ok without adhesion promoter. it is only for bare plastic. you don't need a high build primer unless you have issues, just a 2K sealer is good and it helps to use the appropriate shade of sealer for quicker coverage with your color

Thanks! Yeah, the one piece is scuffed from something, so I'll sand, AP, and prime just that one piece.

Everyone seems to say to scuff the factory primer on these plastic pieces with gray Scotch-Brite before painting. Since I don't have that on hand, would it be okay to go over the primer lightly with, say, 600 grit wet sandpaper instead?

AM.

that would be fine. whenever possible I opt to sand over scotch pads. sanding smooths any imperfections where a scotch pad may not.

I hadn't really planned on using a sealer coat on anything as the paint manufacturer didn't seem to deem it necessary, but now I'm a little concerned about having such different colors under my paint. 90% of the bumper will be the factory-primed black, but the areas I need to sand out scuffs will be covered with light gray 2K urethane primer.

Can I add reducer to my 2K urethane primer and spray the whole bumper with that as a sealer for more uniform substrate color? I asked the paint seller that question, and they said it had not been tested for that, so they couldn't guarantee it would work as a sealer.

Otherwise, I guess I'll have to take my chances that the white paint will sufficiently cover any substrate color differences.

AM.

Although I never got an answer to my above question, I thought I'd post to wrap up this topic.

I asked the same question elsewhere about thinning down 2K high build primer to "seal" (actually just to give me a uniform substrate color), and the advice was... no, it just isn't really meant for such a purpose. Hence, I used Bulldog adhesion promoter anywhere I thought I had bare plastic, applied/sanded high build over some rough areas, and painted it white. It covered just fine.

Another note to "complete the file".... I had one brand new pre-primed aftermarket plastic bumper skirt that painted up beautifully. The other plastic bumper pieces had ridden around on the car for some time. I used Bulldog on all of it just because I couldn't be sure if the primer was still okay. It painted up okay also, but not quite as nicely as the one brand new piece.

Thanks,

AM.

Want to add something I learned the hard way.

I used Bulldog Adhesion Promoter on several areas where I needed to sand and prime the bumper due to scuffs. It worked just fine when used as directed.

Where I ran into significant trouble with the Bulldog was trying to touch up some "sand through" areas with 2K urethane primer underneath. I guess I didn't notice it was lifting the primer in one small area when I applied the color coat, but I had an ugly, wrinkly mess in one little spot.

I had not yet figured out what the problem was in that area, so I re-sanded, got everything prepped (paint mixed, gun ready to go), then sprayed the Bulldog on the affected area only to watch in horror as it bubbled up the paint and primer around the circumference of the sanded area.

Ultimately, I hurriedly sanded the bubbled paint off as best I could, then painted it. I won't be shocked if I end up having the paint flake off in that area sometime in the future, but I couldn't apply Bulldog, and I didn't have anything else.

I have since found other instances on the internet of guys having similar problems with Bulldog and 2K primer. Perhaps it wouldn't affect some primers, but thought I'd throw this out as a caution to others.

Finish Well,

AM.

Bulldog is literally "only" for use on raw plastics. What you ran into is a fairly normal reaction with it. If you look up the msds sheet on Bulldog it's pretty easy to see why it reacts with just about any coating. It's got 14 of the "hottest" solvents that are used in reducer/thinner combos. Acetone, toluene, alcohols, etc., makes it "bite" into the plastic. Guess it will bite just about anything else it touches....