When you try to paint on the texture, recalcitrant polygons appear that do not want to be painted over. Changing the uv does not solve the problem Neither changing stroke or texture settings fixed the problem. I’ve tryed bleed and dither with different preferences, it does nothing. Even more strange is that if you attach a cube to the model, unwrap it, and then delete it, the number of unpainted polygons will change. The model is clean, without errors.
How confident are you that the normals are ok? I’d be willing to bet if you turned the Face Orientation overlay on, your problematic faces would be red, and I’d double down on that bet by saying selecting all and pressing Shift N will assuredly fix your problem
Didn’t open your file, as I’m on mobile, but I’d recognize this telltale pattern of flipped faces anywhere
It’s some kind of a bug I suppose, maybe some old data lingering? Was this model imported from somewhere?
Some faces seem to be assigned to empty slots, even though the slots are not active I don’t think it should even be possible normally…
Anyway, as @Okidoki suggests, select the whole mesh and re-assign the material again.
I don’t think it should even be possible normally…