we repaired my old man's ceffy's factory bumper using a special compound made for repairing plastic bumpers that would cause normal bog/filler to crack when bent. can't remember the name of it, but its sorta like bog, but very pliable. His bumper wasn't cracked so we didn't have to plastic weld it, but basically you plastic weld, then use this flexible compound to fill, and then prep and paint like normal metal/fibreglass.