Paint expert needed
 

Any paint experts here? As some of you know, my son wrecked our Ion and I'm in the process of painting the bumper and fender.

I still have to strip the paint on the fender but, the bumper has been stripped all the way down to bare TPO plastic.

Both parts are used and bumper cover has some bumps and bruises.

Mostly concerned with the bumper should I sand the bare bumper before I start to prime or prime and then sand?