Isn't it frustrating to clean brass over and over again, it just turns dark so quickly. Well a long time ago an Asian friend taught me that it goes black because of oxygen that gets to the surface. Spray it with lacquer right after it is cleaned, and it will stay bright for years.
Spray lacquer is also useful for the difficult task of painting carved letters in wood signs. Carve out the letters. Spray the whole board with clear lacquer, including inside the carved letters. This will prevent the paint from soaking into the wood near the letters, even prevent it from following the exposed grain in the cuts and showing up on the surface.
Let it all dry thoroughly and then simply scrape off the top with a sharp paint scraper. In the third photo you can see the letters on the right were lacquered and the ones on the left were not before painting. You can see how the paint is coming off clean on the right, but I will have to scrape deep into the wood to get rid of the paint that has soaked in on the left.