There's actually no such thing as bare brass - at least in terms of it being untouched from the state it was in when delivered to the horn manufacturers as plain sheet. It's gotta be rolled and brazed, toneholes drawn, fittings soldered on - and by the time all that's done it looks like a complete mess..
Keilwerth once produced a model that was left vaguely unfinished - with solder wipe marks all over the place - and then there's the Inderbinen, left with the finish resulting from annealing.
These esoteric finishes aside the standard practice is to polish and clean the brass prior to applying a finish - be it lacquer, plating or a chemical tarnish...or just a simple wax finish.
The TJ RAW XS starts off with a chemical tarnish - which is then rubbed off.
I'm unconvinced that chemical tarnishes add that much resistance to corrosion given the state that many such horns end up in - but what seems to work pretty well is the sort of tarnish that results from handling, where the crystalline structure of the brass can be see. That seems to be quite stable by comparison, to the point where I would be cautious about polishing it off.