The marine spar varnish would be ideal for the surrounding environment but I will call the manufacturer about its ability to withstand heat. The boilers in an espresso machine get pretty hot but then again I have had some rip roaring fires in that barbeque pit.

Next step it the Wright's and polisher

Spar varnish will remain elastic and soft so it does not crack and peel off.

If you want actually stop the oxidation (rusting) then something such as POR-15 is what you want to use. Use POR-Patch to fill the inevitable craters before coating with POR-15. If exposed to UV then overcoat with one of the POR-15 uv proof products. They have colors and a clear, either of which are UV proof. Some of their coatings are also for high temp applications. The POR-15 will not peel off or crack. These products are used extensively in antique car/truck restoration.

Thanks for the tip!