I like to top off the POR15 with either an undercoat or epoxy, just to seal it all up.
Re: the bolts—if it’s just the heads, you can lightly brush them (not enough to impact head shape), and POR15 them too. Or you can buy replacement bolts, if you’re really worried about it. The small fasteners on my Hoser-Mobile are definitely showing the effects of the north, but the large fasteners are all perfectly usable.