Do I need to remove the stock bumper to install this?

Yes. You'll need to unbolt the existing bumper cover and reinforcement, then mount this bar at the same factory anchor points before reinstalling the bumper assembly. It's a straightforward swap using your vehicle's service manual as a guide.

Will this work with an aftermarket rear bumper?

Only if your aftermarket bumper uses the same factory mounting structure. Test-fit before committing to installation. Some custom bumpers have different anchor geometry and won't align properly.

Does this need paint or primer before I install it?

The bar installs behind your bumper cover, so finish is your call. Many builders leave it bare since it won't be visible. If you want protection against rust, prime it before assembly.