I have a model that I unwrap using "smart uv project". None of the UV's are overlapping but they are squeezed somewhat close together, even when I set the island margin to 1. I then click "Face selection masking for painting" in the Texture Paint mode, and it seems like it's going to work-- I can right-click on particular faces and paint onto those faces.
But: when I paint on faces, the paint goes past the lines of the UV, there's spill-over into the in between space. Face-selection isn't setting up a mask. And if another UV is there, that means other parts of the model are getting paid because of that spillover that I never wanted to be painted. This seems like it will be an issue even if I unwrap it manually with seams and what have you since the issue isn't my unwrap-- it's that when I try to paint, it doesn't stick to within the lines.
(Larger goal: I want to go face by face and add colors to the different faces, ideally using some kind of fill tool-- I think I could use different materials, but that would be disadvantageous for later use in Unity(?).)
(My issue is also detailed here).