Many aftermarket ground effect or bumper kits that are constructed out of fibreglass will come without an opening for the exhaust pipe to fit through. This is because many aftermarket pipes are odd shapes or in odd positions. Cutting an opening in a fibreglass bumper is fairly easy, but will involve more than just cutting it. Fibreglass must be sealed after it is cut or sanded to stop water from eventually penetrating the fibreglass and causing delamination. All fibreglass materials used on cars can be found at your local auto body supply store.
Dry-fit the bumper on the car such that the exhaust pipe presses up against the inside of the bumper. Make sure all the bolt holes are lined up on the bumper, and trace the outside of the exhaust pipe onto the inside of the bumper using a Sharpie marker.
Put on a respirator and safety glasses and drill the hole in the bumper using an electric drill with a hole saw bit that is 1/16 of an inch larger than the diameter of the exhaust pipe. Sand the edge of the hole smooth using 100-grit sandpaper.
Mix a small amount of fibreglass resin that contains a sealer with 1 per cent catalyst in a small bucket. Stir together with a paint stir. Apply the resin to the edge of the hole using a paintbrush. Wipe away any excess resin using a rag and let the resin harden.
Install the bumper onto the car carefully, tightening down the screws or bolts so the fibreglass doesn't fracture.
Use a respirator and safety glasses anytime you are cutting or grinding fibreglass.