After having to do very extensive touch-ups every spring, I'm ready to try something new...
The problem is all the sand & salt that gets put on the roads during the looong New England winter. It basically sandblasts the cross bars on the underside, then adds salt for extra-quick rusting.
This year I am getting them professionally sandblasted, then painted with a thick coat of POR-15, then top-coated with Rustoleum textured black (the POR-15 is not UV stable, so it's fine all by itself for undercarriage parts where the sun never shines, not so much for the tops of the sliders). Finally, I am adding small ABS plastic guards to the cross members on the underside - where they take the most hits from sand and gravel.
We'll see how they doing come next spring.
I've thought about some sort of plastic gaurd to the high wear areas that tend to get sand blasted off. Post pix when you get yours done!