There is a clear lacquer that is used to seal PC boards, its a spray can and I am sure its acrylic. I have found this to be good for surface covering. Alternatively you can use Cellulose clear lacquer, 'Airplane dope'.

I have had good results with both on steel and brass.

Clive

I use Rustins metal lacquer for keeping a bare metal appearance but protecting from rust. It goes on like water without leaving brush strokes but has kept anything lacquered in the cellar rust free for more than three years now.

Andrew.

I'll second this – I made a gunmetal and steel medieval mortar about 6 years ago, both metals showing signs of corrosion after a few weeks on the mantlepiece. I cleaned it up again and coated it with Rustin's Metal Lacquer, since when it's sat on that same mantlepiece, sometimes being picked up for examination by sweaty hands, and it still looks like it did the day it was lacquered.