Simple Texture Painting Problems

Hello! Today I was trying to paint some grunge onto a metal surface, or something almost like the metal had partially chipped off, similar to this image below.


However, I only want the imperfections in certain spots, and of course I want these areas to have a diffuse texture, while the metal areas are still shiny. I am running into two problems however.

  1. The area I want to texture paint is completely white in texture paint mode and I can’t paint on it.

  2. I am unsure how to set up a brush for this using a regular image.