I think a coat of clear powdercoat would work if you like the look of bare steel. From what I've heard, automotive-type clearcoat will NOT work--rust will form underneath the finish. I suppose you could also just wipe the naked, unfinished steel frame with Boeshield and a rag every now and then--that would probably work fine.
I'd be careful about etching the frame, though--it just doesn't sound like a good idea.
I leave further comment to someone who actually knows something.
JV