Wheel arch rust

Mercedes 300E 1996. I have a small patch of rust coming through at the base of each wheel arch that I want to treat. This is at the join of the plastic
skirt that runs along the length between the two arches, I suspect the rust areas are hidden under the plastic. How should I treat this rust and if necessary, how do I take off the plastic skirt? (prefereably in one piece!)
Charlie Brown

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