Hi,
I have unwrapped a building and assigned a seamless brick texture to its walls.
Now I would like to paint some dirt on the entire walls but I don’t want the dirt to be repeated (I don’t want to paint on the seamless texture but on th entire mesh).
What’s the best solution to achieve that?
Thanks for your time and sorry for being such a noob: it happens to all of us sometimes! 
You can create a new set of UVs that will work as you want with your new dirt texture
You can create a new texture slot and paint onto this new texture with the newly created UV map
Your can use a blend mode to mix the brick and dirt textures
You can set this dirt texture not to repeat
You haven’t said which renderer you are using as the material systems are quite different so how to do the above will be different