Painting rust material onto a sign?

Hey, I’m trying to get a more realistic effect for my new project, but I’m not sure how I can get a rusty texture onto an object such as…a rusty door or sign…
How would I do this? I’m pretty much lost at this point…

Here’s a screenshot of my work so far.

I’m trying to paint the areas that are green and red.


You mean like loading up a texture and then and painting it on?
You might not even have to UV unwrap to do this -