Scout Bumper Protector

My current Scout rear bumper has a black sprayed resin type coating but with 5 years of use has various marks and scores. In order to replace this eyesore I purchased a silver colour plastic bumper to cover this.

However the bumper is about 20mm too short and it means the original black coating still shows on either side.

I would appreciate if any one can advise on how to remove the original black coating.