Before you even seal them, make sure you buy some spray rust-preventive up under the fenders where they have all the little spaces in the bracing underneath. Spray them heavy and let the coating run into the places where you can't see-like the brace under the hood latch. The best stuff out there is the same stuff we use at the body shop when replacing a door skin on the outside of passenger car. There are many different brands, just ask a body shop supply center for their idea. Just depends on how much you want to spend. I just got done putting new fenders on my wife's TJ, and I used panel sealant by U-Pol under them, then base coat clear coated after they dried for 24 hours. TJ fenders suck...crappy design that holds all kinds of crap inside the bracing. I have had a pair of aftermarket YJ fenders on my 95 Rio Grande since 2003, and they haven't even started to show signs of rust, they were better designed in the bracing areas...less places for crap to lodge into empty spaces.