At some point, Dallas Railway and Terminal decided that brass window sash were the preferred option for their streetcars, using them on their Peter Witt cars and some other pre-PCC equipment. I have a 1927 Northern Texas Traction double-truck Birney that was acquired by DRT after NTT got out of the streetcar business. I don't know if the window sash are factory or something that DRT installed but they are brass. Does anyone have any experience and tips for painting brass? I am replacing broken glass and trying to get the car weatherproofed and will be repainting the sash. The car is in the DRT victory paint scheme, which I will probably maintain for the time being. I think it is a much more attractive scheme than the NTT kodak yellow and black.

I’m not aware of any special prep requirements for painting bare brass. If you’re positive that you’ll never want to polish and lacquer the brass in the future, I’d scuff sand it lightly to give the surface some “tooth” then prime & paint as usual.

The only thing I can think of that may give you a problem is compatibility with any kind of coating that may be on the brass already. When in doubt, test a spot first.

Googling it, I've found some recommendations to use an etching primer.

SEM primer works great on brass. If you're never going to paint it, then either lightly abrasive blast or rough it up first and make sure it is clean.

This is not precisely related to paint, and not in the historic preservation industry, but in case you are wondering what's in self-etch and how it is intended to work on a substrate, look at this reference

https://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-64402015000100003

More information from the same general industry, but more directly addressing coatings over metal:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5296570/

Is there a reason to research expedient ways of stripping a self-etch-primed coating should anyone want to go back to polished brass pieces?

You might consider doing "nothing" to the brass and just allow it to develop a natural brown color over time.
T7

I don't think any research is needed. The basic chemistry of self-etching must be simply to dissolve the zinc atoms at the surface of the piece, leaving a rough surface of copper. The only way to get a fresh brass surface would be heavy buffing to remove the surface layer which was converted to pure copper.

Self-etching primers for brass is probably the only useful application for the corrosion process of dezincification of brass, which can cause plumbing fixtures to fail.

Did either Dallas Railway and Terminal or Northern Texas Traction paint the brass? If neither did, then not painting it probably would be more appropriate restoration.

Or clean it and dress the surface to the finish quality you want, then chemically treat it to accelerate the natural 'patina' formation...

Liver of sulfur (mixed potassium sulfides) is available on the Internet for use in developing patinas on copper and copper alloys like brass. I used it in my high school art metal class almost sixty years ago.

Just so.

One of the 'unintended consequences' of the Clean Air Act and subsequent pollution control is that certain types of 'patina' no longer form as quickly or definitively as they once did. There is a famous example of architecture at Cornell with a facade in Cor-Ten which was known to form an oxide layer quickly to protect the surface against further corrosion. Problem was, in the air in Ithaca that didn't happen, and all that happened was a runny, rusty mess. I believe they had to simulate 'air pollution' on the surface chemically to get the magic to happen.

One very dramatic effect is formation of 'natural' sulfides. Growing up near New York, it was understood that silver would start to blacken, even in weeks, and might require a 'dip' before polishing for use. That doesn't happen any more, and now with ULSD standards, I've almost forgotten how that used to be. The parallel here is that 'polished brass' isn't supposed to be lacquered, it's supposed to be maintained, and if one of the first things that 'went' in the financials of a trolley line was regular brass polishing, it would be logical in air full of coal smoke to see 'patina' of sulfides form to a consistent, and I believe reasonably self-limiting, layer on those surfaces.

As a historic-fidelity 'aside', I'd also expect different patina to form in different geographical areas, and serious preservationists might want to 'match the hatch' for the region and time a particular car ran... especially if the result were inexpensive and relatively maintenance-free.

The DR&T wartime paint scheme was willow green, cream window band separated with a red stripe and a pearl gray roof. The window sash were definitely cream. Bare brass would be inappropriate as well costly to my efforts because if the copper mice figure it out, the windows would be liberated. They will definitely be repainted and reglazed before being reinstalled.