Cracker,
I used a product called "Gator Guard" by Eastwood Co., see my photo gallery. Did it after the paint job, have to really mask off everything. Its bed liner material. The thing about undercoating is that
oil, fuel, and other products will disolve it. This stuff is tuff and should do a great job protecting the fenders. DupliColor makes their bedliner material in spray cans, Qt's and Gallons. I used it in the trunk area and all under the car, works great.