The frame on my ironhead sportster was done using POR15 chassis paint (direct to metal, no primer). It's a typical rust "encapsulator" and does provide a very solid shell over the frame. However, there is no "touching it up" or blending-in blemishes. It doesn't sand. It doesn't buff. It's almost more like a very thin but hard plastic shell. I'm sure the next person to take that bike apart will curse me as it's probably going to be tough to remove it all.
It was pretty easy to apply with a brush and was self levelling. I'd use it again if I was re-doing it but I'd be more careful during assembly to prevent scratches, as any touch-up is visible.