A lot of the early brushed aluminium stems and cranks had clearn coatings. I know because I sanded off and polished enough of them for my own bikes. Bare brush sanded steel parts definetly would have been clear coated or made from stainless tubing as it would be rusted before you could even buy it otherwise.

As for a nice clear coat for steel you got me. I tried half a dozen and when applied to the nicely brush sanded steel or alloy they all turned a dull grey as the coating filled the shiney cracks and ruined the optical charactarisitics that produced the specular reflections that give a nice brushed finish it's look.

I understand that nitrocellulose lacquer is supposed to be good. But I've never seen it in rattle cans.