New fender on '78 KZ- what to put in the underside

I got my fender bent by some moron in a car. The rear fender on my KZ-650 (1978) got folded down to the tire. I was luck enough to find a new (old) one on e-bay, but the underside was understandably dirty and had a fair amount of surface rust. I cleaned off what surface rust I could with a brass brush though some remains.

Before I mount this back on the bike, what should I coat the street side of the fender with? Paste wax? Doesn't seem like that would last very long. Clear coat of some kind? It would have to come in a rattle can and I'm not sure what to use. Rust proofing undercoat? Rustoleum in a jet black?

The bike is original blue.

TIA,
Tom

My front fender has been treated w 3m rubberized undercoating - it's what I had at hand, so that's I used. I've used bar-b-que grill paint as a rust preventer on the underside of other parts.

Thanks! I appreciate the tip. Will the 3M stuff stick if I don't get the part totally clean of rust?

Probably. For sure you want the part clean & dry w all loose rust removed. Something else.... I completely forgot that I treated mine w DupliColor Rust Fix before applying the undercoating - it's one of those rust converter rattle-can sprays. And I refurbished a rear fender that wasn't too bad by cleaning it, etc, then spraying the underside w bar-b-que grill paint (as a rust treatment), followed by the same 3M undercoating.

Rust treatment & prevention - pic

Whatever you decide to use you really must remove all the rust first. Here's something I've used with great success. Soak the underside of the fender good and keep it wet with this stuff for a couple hours and the rust will be gone. It may take a couple treatments. Then wash & dry it and paint. This stuff is made for firearms. It doesn't seem to affect the good metal.

Click on "gun blueing" then "metal finishing" then see Birchwood Casey Blue & Rust Remover Product 16125 -BR1

www.birchwoodcasey.com/sport/index.html