I have rust spots on the rear fenders. I will fix the lower radiator core support but I don't think I will fix the fenders. The car is 10 years old and if I just keep up on everything to keep it running I think it's not worth it to fix the rust.

 

Just sand away the rust and paint over it with a primer that has a rust inhibitor.

How large an area are we talking here? Got any pictures?