I've cast thousands of lost-wax brass parts over the years, and also both built and modified brass structures and rolling stock/engines and I never (NEVER) found it necessary to "etch" any of them to get paint to stick to them.
The trick is to make sure to get all of the waxes and oils off of the brass part. I first wash them in a mild, warm, soapy water solution (I like Dawn) brushing them with a soft, clean toothbrush, and then I blow dry them with clean, dry compressed air, holding the mounting pin or sprue in a hemostat so they don't blow away. Then, still holding the part in a hemostat, I liberally brush them with clean Bestine (Heptane) and that takes about a minute to evaporate. At this point, they are ready to paint. I have a lot of hemostats, and I hang the parts attached to the hemostats on my spray booth on hooks I have there for that purpose. I can hang a dozen hemostats clamped to cleaned parts, painting them one at a time.
I find that enamel based paints work best with brass. I like Scalecoat II the best. I apply a thin covering coat of paint to the brass object and let it dry for about ten minutes, then look at it and see if the paint has fully covered everything. Light colors usually need another coat, whereas blacks, browns, greens and blues cover in one coat.
After making sure there are no pools of wet paint on the brass object, I place them in a pre-heated toaster oven at around 175 to 185 degrees. I use an oven thermometer to tell me when the oven is ready. I bake them between 10 and 20 minutes, then remove them, hanging them again on my spray booth hemostat hooks and let them cool.
This process works GREAT for me, and the paint is always durable and never, ever peels or flakes.
If your newly painted brass object still smells of paint thinner after it's dry, you need to bake it longer. The paint is cured when the paint thinner odor goes away.
Just sayin'
Cheerio!
Bob Gilmore
