Will this molding work with aftermarket bumpers or bumper covers?

This is engineered for factory bumper geometry. If you're running an aftermarket bumper or cover with different contours, test-fit before final installation to confirm alignment.

Do I need to paint this before installing it?

Depends on your finish preference and the condition of your existing trim. If you want color-matched results, paint prep and finishing will be necessary. Otherwise, it can be installed as-is.

What's the best way to attach it — adhesive, fasteners, or both?

Mounting method isn't specified in the product data. Check your vehicle's service manual for the factory attachment points, or test-fit the molding first to determine whether fasteners, adhesive, or a combination works best for your install.