Yes, you are correct. The surface must be clean from grease and oxidation. A soapy solution will work just fine to remove grease, and vinegar will remove oxidation. If you didn't solder the parts, you can clean them with Windes. It works OK. Usually, I clean the surfaces with sandpaper before removing them from the frets.
Also, you should experiment with a blackening solution. some brass alloys are different from others and will require some experimentation with proportion water and blackening solution. I use Brassblack and usually do 1 part blackening solution diluted in 5 parts of water. it is a slow process, but it will adhere to the surface better.