Will this reinforcement bar work with an aftermarket bumper?

Compatibility depends on your aftermarket bumper's mounting structure. If it uses the same factory mounting points, you're good. If it uses a different bracket or frame setup, test-fit before final installation.

Do I need to prime or paint this before installing?

The bar comes ready to bolt on. If you want a finished look that matches your bumper, prep and paint before install — easier access that way. Otherwise, it can go on as-is.

Does this add any weight that affects handling?

It's a lightweight reinforcement, so the difference is minimal. You're not looking at a noticeable impact on how your Jeep drives, but you will feel the added rigidity at the front end under load.