I recently had my rear arches repainted due to rust. I had the car resprayed just under 2 years ago and the guy who was supposed to remove the rust from the rear arches (only looked tiny) didn't do a thorough job of it. He welded a small patch over the affected area but the actual rust was caused by rot in the inner wheel arch rusting through from behind. It came back through a few weeks ago.
This time I repaired it myself. I cut the outer arch out in small sections till no more rust was behind. This meant I had to cut away sections of the inner arch back to clean metal, then weld plates in. I then welded repair panels to the outside. I went to town on the rust removal, making sure I removed the lot. All joints were sealed and the inner arches have been sprayed with engine oil then waxoil to prevent any rust returning... I got the paintwork done by a guy for a very good price. It doesn't have to be expensive if you can do the prep yourself. It is also very satisfying knowing you have got rid of a nasty thing such as rust properly.