*Note to readers: Im a newbie to automotive refinishing. I grew up around it, helped with remedial repairs but never anything too serious outside of spraying single stage.*

I purchased a pre-owned rear bumper cover for my truck, but it's in poor condition with deep scratches that expose the plastic in various spots. To address this, I started sanding the scratches with 180-grit sandpaper, inadvertently exposing the plastic in some baseball-sized areas. After following up with 400 grit and a little filler, I smoothed out the remaining scratches, applied a black Transtar 4613 primer from a spray can, and the repaired section looks decent. However, I'm worried because I skipped using an adhesion promoter this time. My previous attempt using adhesion promoter on a similar repair caused overspray onto the base coat, making it appear wet and somewhat melted and causing wrinkles in anything sprayed over top of it. While the local paint shop suggested that Transtar 4613 has properties for plastic adhesion, I'm hesitant to use adhesion promoter again due to its adverse reaction with the base coat. Any suggestions on how to handle this?

Besides the two exposed areas the covers still in the OEM finish WA8555 GM Black.
. I’m planning on sealing the entire cover with PPG shopline 337 primer sealer before base coat.

JPS0284 wrote:I'm worried because I skipped using an adhesion promoter this time

Don't sweat it. I've painted thousands of bars that have been repaired like yours and never used adhesion promoter which I reserve only for use on bare plastic that is of known good quality and has not been sanded or maybe can't be sanded due to moulded surface patterning or similar. The sanding provides more than enough surface for primer to adhere to.

JPS0284 wrote:My previous attempt using adhesion promoter on a similar repair caused overspray onto the base coat, making it appear wet and somewhat melted and causing wrinkles in anything sprayed over top of it.

Application too wet, allowing solvent to sit on the surface and soak in to any exposed edges. AP needs to be put on very, very thin. A 5 litre tin of it lasted me over 10 years, doing up to 3 bars a day.

JPS0284 wrote:applied a black Transtar 4613 primer from a spray can

Not ideal but if this primer is compatible with the base/colour coat that you'll be using then it will be fine.

JPS0284 wrote:I’m planning on sealing the entire cover with PPG shopline 337 primer sealer before base coat.

Why? Other than the damaged area, which you've now repaired and primed, is the paint damaged on the rest of the bar? If not, just sand with P800 and go straight to base/colour.

Don't make extra work for yourself that isn't necessary. I'd put extra effort into making sure that the surface of your repaired area is spot on perfect. Anything less, under black, will be visible. Guide coat it and block it out. "Looks decent" isn't good enough.

NFT5 wrote:

JPS0284 wrote:I'm worried because I skipped using an adhesion promoter this time

Don't sweat it. I've painted thousands of bars that have been repaired like yours and never used adhesion promoter which I reserve only for use on bare plastic that is of known good quality and has not been sanded or maybe can't be sanded due to moulded surface patterning or similar. The sanding provides more than enough surface for primer to adhere to.

JPS0284 wrote:My previous attempt using adhesion promoter on a similar repair caused overspray onto the base coat, making it appear wet and somewhat melted and causing wrinkles in anything sprayed over top of it.

Application too wet, allowing solvent to sit on the surface and soak in to any exposed edges. AP needs to be put on very, very thin. A 5 litre tin of it lasted me over 10 years, doing up to 3 bars a day.

JPS0284 wrote:applied a black Transtar 4613 primer from a spray can

Not ideal but if this primer is compatible with the base/colour coat that you'll be using then it will be fine.

JPS0284 wrote:I’m planning on sealing the entire cover with PPG shopline 337 primer sealer before base coat.

Why? Other than the damaged area, which you've now repaired and primed, is the paint damaged on the rest of the bar? If not, just sand with P800 and go straight to base/colour.

Don't make extra work for yourself that isn't necessary. I'd put extra effort into making sure that the surface of your repaired area is spot on perfect. Anything less, under black, will be visible. Guide coat it and block it out. "Looks decent" isn't good enough.

Other than the few deeply scratched areas and some small stone chips the rest of it he covers in pretty good shape. I’ll stick to what you suggested, take my time and focus on getting the rough spots looking the best I can. The plus side of this is there no immediate rush to get it done asap, it’s merely a personal project to learn on and eventually get to show off someday. Thanks for the advice I really appreciate it!