I haven't read every post in this thread, so if someone has already made
this suggestion, my apologies.
If I wanted to coat something steel so that it would last forever, I
wouldn't even consider a paint or a clear coat. I would have it nickel
plated at any shop that does chrome plating.
Nickel is a fairly hard metal, so even if these hinges have wearing
surfaces on them where they rub against other metal surfaces, the nickel
plating will outlast any other coating on that same surface.
Electroplated coatings are probably the most durable coatings you can
put on metal.
Nickel is an important metal in making stainless steels because, just
like chrome and many metals, it forms an oxide film over itself that's
extremely impermeable to air and water. So, the formation of that oxide
film protects the underlying nickel from further oxidation. It's so
thin, in fact, as to be invisible. That's why nickel and chrome don't
appear to "rust". The oxide film they form is only a few dozen atoms
thick, and light passes through it almost unaffected.
(Gold and platinum don't form an oxide film AT ALL. Most metals form
some oxide film over themselves.)
In metallurgy, there's a general rule that when you mix metals, the
properties of the alloy will be largely determined by the properties of
the parent metals. So, if you mix a lot of nickel and chromium into
iron, you get a steel that forms an protective oxide film over itself,
just like nickel and chromium do. That's exactly what the various kinds
of stainless steels are; iron with enough other stuff in it to make it
behave like that other stuff.
So, if you can't buy stainless steel parts, the best alternative would
be to have the ordinary steel parts nickel plated to give them the same
self healing oxide film that stainless steel has.