New unpainted brass coating?

As a first guess, likely lacquer

As a second guess, sometimes the Koreans used something impervious to lacquer thinner or paint stripper.  The stuff never comes off.

In that case, just how do you prepare it for painting - sand blasting?

If the clear coat is in good shape I just wash the model in warm soapy water to remove finger oil due to handling.  That clear coat is generally pretty tough and serves as a good primer on the bare brass.   If the clear coat isn't in good shape, or if I've done re-detailing with a torch, iron, or resistance soldering  unit, I'll clean the messed up areas using aluminum oxide in a blasting cabinet before washing it.   I wouldn't use sand or other blasting material as it may distort the brass -  I made that mistake with an old US Hobbies express reefer body.

Ed Rappe

Thanks for the info Ed.  You mention the clear coat is tough, but also works well as a primer.  I'm sure this guy will want to know about it's adhesion properties?

Clear-coat or no clear-coat, I think it is a good practice to apply a light gray automotive primer on what is very often a very old Clear-Coat. I have had mixed results when painting directly on the Clear-Coat.

No later than a few days ago, I had to strip my painting on an engine because I forgot to apply primer. When I tried to bake the new paint, some spots developed and were particularly ugly. After stripping the new paint, I realized that the "old" clear-coat had re-acted negatively to the new paint and probably to the baking.

For Primer, do not use the junk called Rustoleum sold in Home Depot or Lowe's as it is double coating and will turn your model into a magma of primer. Instead go with Plasti-Kote T-235 (hard to find) or some fine and sandable automotive primer.

Yves

Hi Yves -

Actually, your experience might work for me.  I'm looking for a military modeler to do the painting as they're very experienced at producing a well-worn rusted look, which is what I want for my industrial Plymouth.

Next I have to decide if I'm willing to bash-up a new brass engine just so it will look appropriate to  wear such a paint job.

A NG Plymouth at East Broad Top - a bit more dilapidated than what I'm looking for.  After all, there's an electric motor under the hood, so I can't remove any panels to display a gas engine.  I do kind of like the gas tank - or is that air? - protruding through the front wall of the cab, and might try to replicate that.

A mine engine, with more of the sort of finish I'm looking for.

I vote for stripping in lacquer thinner then blasting with 220 grit aluminum oxide.    Wash in a tub of hot dawn dish soap, rinse in hot water and air dry.

paint soon after.

Well, me too, if lacquer thinner cuts it.  Go very easy on the air pressure.  I use 35 psi max when blasting old Lobaugh locomotives, and no more than a 45 degree blast angle.