Company Altex does a marine protectant coating called RIPO.1 or 4 liter tins.
Toxic- lots of Xylene, so you know it's good.
goes on really thin,so it creeps between crack and panels. It's clear-ish,does harden up eventually, won't wash off in the rain, seals up seriously rusty bits and keeps bare metal perfectly fresh. mmm-mmm.
I'm pretty sure you can paint over it too when the time comes.