IMHO, pull the fenders. I thought mine were fine in behind there, no rust, no problems. Boy was I wrong. Pulled them off, and half a 5 gallon bucket of dirt, rust, and "other materials" fell out. It also exposed quite a bit of otherwise hidden rusting metal, which, if left to its own devices, would have most likely crept out of the little hole they were in and showed up on the paint or under the floor. Then you get to paint it all over again!