This is a prime example of a "Cut & Buff" job. The rear bumper, lets just hope you still have some clear left otherwise things are going to get messy in a hurry.

This is just normal oxidation and it happens to all cars. On the worst parts, you can wet sand with some 2000 grit then move to 3000 grit and then polish with a rubbing compound like what I used in the headlight video (I used paint compound on those lights lol). For the spots that are not bad enough to need sanding, you can just use the compound to bring the shine back up and then hit it with a good carnauba to keep the shine.

If at any time you are wet sanding or buffing and you notice blue in the slurry or on the pad, you have cut through the clear and the only option here is to wet sand the entire panel with some 1500 and apply a new coat of clear. It sucks when it happens, ask me how I know lol.

 

You can, just remember the base coat is not going to be as strong as it was new due to radiation and UV rays. So even though you can clear over it after prepping, you may cut through to the primer coat.

 

I have been doing youtube videos helping people since about 2011. When I started, I never showed my face on camera. Then in 2014, I said screw it lol. It's fun for me to help people and make these videos. I need to get out there and make more.

As for your car, just keep at it, but stop if/when you hit color. Color is bad. My car has no clear on the back 2/3 of the car. I am about to paint the car (on the very cheap) and might just get that on video.