Hello all! I'm new here and a hobbyist about to paint my first complete car, my son's car. I bought new bumpers for it and I have never delt with prepping plastic bumpers before, and was unaware they can come with primer on them now. Anyways I started scuffing with grey scotchbrite on one bumper and would wipe down with alcohol. The primer started to soften and come off. So at this point I'm looking for the best chemical to strip this primer off so I can start from fresh and use Polyvance. I've tried alcohol and slow reducer and both are taking forever! Any help and suggestions greatly appreciated.

Chemical strippers and plastics are not a good combo. Most of us sand paints on plastics and even at that you have to watch it about distorting the surface. If strippers are aggressive enough to remove paint quickly that means they can also melt the suface profile. Keep in mind if you do end up at bare plastic on this you are going to have to use an adhesion promoter before moving into primers....

Why do you want to take off a primer that's put on at the factory, probably better than you could ever do?

Scotchbrite the whole bar and give it a coat of epoxy. Then your topcoat(s).

I wasn't planning on removing it, but the alcohol softened and smeared it around so now I don't really trust it.

Over the years I've bought hundreds of new bars and every one that was factory primed came with instructions, either on a sticker on the back of the bar or on a sheet in the bag or box. Depending on the primer used the instructions varied - sometimes just wash, sometimes lightly scuff and sometimes scuff and degrease. Whatever, I found following the instructions always worked.

If alcohol has damaged the primer then it's most likely waterborne, although you do realise that you shouldn't use straight alcohol, rather a 50:50 mix with water.

Where you've damaged the primer with rubbing and alcohol you can sand smooth if you want, with P400, by hand. Grey Scotchbrite would work, as I said, but the important thing is that you don't damage the surface further, whether with paper that's too coarse or by using a machine and changing the shape.

Then, as I said, epoxy as a sealer and your topcoats.