I use vinegar full strength 2 or 3 hours will do it. Depending how dirty they are more time may be needed.I have some detail parts that are brass and want to prep for painting. I've heard you can use vinegar to clean and etch them.
How long to soak in the vinegar?
Vinegar is also good for cleaning your plumbing in the house like the faucet screw-on strainer or the shower head anything that has that caked on hard water.
I will put them in full strength vinegar covered completely for 4 or 5 hours then just rinse them off it works great.
A used toothbrush helps after you soak them, a soft bristled brush.
I save all our used toothbrushes for cleaning.
Some brass articles are lacquered coated the vinegar wont work good on these. Something like a door knob. Sometimes these are lacquer coated to protect the finish.
Train detail parts should not be.
You also can make a paste to clean it using equal parts vinegar, salt, and white flour. Then apply it to the brass and let it sit, don't let it dry then use the toothbrush to scrub it then just rinse it off.
If you let the paste dry it is a little harder to remove.