POR 15, the stuff works great.
I use it alot on car frames, where anytime i'm under and I see a spot of rust I wire brush till nothing is loose, por 15 primer, followed by rubberized frame paint for aesthetics.
On a bike, i'd wire wheel rust, por 15, then paint of choice. As mentioned if you really want long term paint you'll need to redo it. Powdercoat is more durable for offroad environment.