What year/model is it?
Some people have found perfect paint underneath, some people have found a complete mess!
When I took mine off, they just needed a really good T-Cut and they came up nice. But There's some horrible looking black plastic plug holes to the side of the arches that some of the trim clips into. To smooth this over would be alot of work.
That's for my 90 spec Driver with small bumpers. Don't know if it's different for big bumper models..
To remove the arches the best method I've found is to take the wheel off, starting from the top center rivet, drill into the center a little bit (You don't need to drill all the way through.. Just maybe 5mm down. If the disc bit of the rivet doesn't pop off, just prise it off with a flat head. Then just get a smallish crosshead screwdriver and a hammer and just whack the rivets through.