I have just found the same problem with my small bumper (F) 5 door. All I had was a small amount of paint bobbling over the n/s/r arch.
When I took the arch cover off I found rust the size of a tennis ball. So I decided to take them all off, every arch had some degree of rust.
I have now taken off the side trim and arch covers and filled all of the holes and I’m now in the process of sanding it all down.
I have managed to get some ¾ spats for the rear arches which will save having to filling the holes around the rear arch.
Personally I think they look smart without the trim on small bumpers but not so sure about big bumpers.
Let us know how you get on.
