How do they do it?

Acid dip, then clear coat.  Older models were shot with gold paint.

John,

Used to be they shot a coat of clear over unpainted brass.    

Simon

I always suspected that they used acid, but I was never really sure. Would you know what kind? Sulfuric? Nitric? Hydrochloric? What concentration?

J

Whatever it is, it is called "bright- dip" and you should probably get it done commercially.  Only takes a couple minutes.  Then you do not have to have acid laying around.

For painting I recommend bead blast, at 35 psi, and at an angle.

all you ever wanted to know . . .

http://www.musictrader.com/picklingbrightdipping.pdf

Plain old household vinegar is mildly acidic, and mixed 50/50 with water will do a nice job removing rust. I never tried putting it on brass, but I'm guessing it would work! A lot less dangerous than stronger acids and easy to obtain!

Simon

I've done an overnight dip of a brass body in a tub of water with a whole can of bar keepers friend.  Came out shiny for painting.

I am with Michael concerning Bar Keeper's Friend. It has performed wonders for me at cleaning very tarnished old Max Gray models.

Yeah, me too, but the original question was how do the manufacturers do it.  They "bright- dip" it.