You can paint it with just about any clear varnish but the trouble is you want the surface to be nice and shiny first, so consequently the varnish has nothing to key to and so tends to easily chip and scratch. A lot of varnishes tend to discolour with time, which tends to be a little bit more noticeable when over polished brass than it is over wood and the surface does dull with age. Plus you have also burned your bridges, so to speak, as when the varnish chips and water gets underneath, or when the varnish dulls or discolours you have to strip the varnish off before you can repolish the brass, which is a real pain. Consequently the brass never looks quite as good as you want it to.
I know its not what you want to hear but the best way to treat brass is to polish it and then simply re-polish it as and when it needs it. This has the added advantage of developing a lovely patina over time and the brass will actually take very little to recover the shine one it has been originally polished.