Anything that is UV rated will work,i would put some good quality primer on before the paint for a good adhesion.
Unfortunetly the BEST way would be to strip the whole thing to bare metal,then Epoxy primer and rhen paint for the longest lasting job,but unfortunetly TRUE Epoxy primers are a 2 part ,so it would have to be sprayed with a paint gun/compressor.
Any paint job is only as good as the adhesion you get,that's where good primers come into play.
POR-15 is ok.but it must be totaly to metal and very clean,it doesnt bond well to anything else but a metal surface,but anything you use must be cleaned and sanded for a mechanical bond anyhow,just not totaly stripped for some other stuff.
Go with a good etching primer,or even a can of true Red Oxide primer(NOT THE ONES THAT ARE JUST A RED OXIDE COLOR) and then a good UV rated top coat.
After the final finish..you can buy a good silicone or teflon based paint sealer and apply that on it to help keep the paint from drying out in the hot sun in Mexico(i live in southeast Ga i KNOW the big problems of sun baking paints and fading them fast),this also protects it from moisture and mud and will clean easily.
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