Thinking about it a bit more, since the rust creates a hole thru the chrome, there would probably be a little ring around the rust pit, but on the bottomside of the fender, who cares. Its not sticky when applied, does get a little bit as it dries, but once it's dried, the oxidation is stopped and the treated part can be rinsed with water, and refinished (or not) to your preference. Treated spots won't rust again, but it won't prevent rust from happening when there is new damage to the metal coating (paint, chrome, bedliner, etc..) that exposes bare metal. A good wirebrushing and ospho treatment followed with a coat of bedliner and that fender would prolly outlast any fender made today. On the other hand, unless you've got holes thru the fender, it would seem the chrome (which is usually a bit lower quality on the bottom side) has done a pretty good job of protecting the metal for the last 40+ yrs, bedliners prolly overkill. Then again, you could do the whole bike with bedliner (a friend of mine painted his K-5 Blazer with it)....seriously off road mean and nasty lookin....just don't rub no bare skin on it....
