I can paint on a layer mask for a 3D layer, or paint on the 3D layer without any problems.
What problem are you seeing, exactly?
Ok, thank you very much mr. Chris Cox.
As i understand i can use different layers and masks, in document with model, and then merge it and it will project on mapping texture. But i think that it a bit uncomfortably. Because, for example i have model of gun. I want to paint rust in some places of it. How i usually do it. I make new layer filling it all by texture of rust, creating mask, filling it black (hide) and then painting on mask by white color in places where i want rust. And i want do exactly same but on 3d model, because it more simple and comfortably.
But as i understand i need to create new layer and layer mask in document with model, and paint all there, and then merge.
In this case i have 3 problems.
1. I can't rotate model before i will merge.
2. My texture of rust for example will projected on texture not correctly with distortion (as compared if i will paste it on mapping texture)
3. When i will marge this layer with layer mask i will lose my mask.