Quote - Just to enlarge on Paul's thought, the mask needs to all be within the perimeter of the lip material zone.

Just in this case, because the lip polygons are completely joined to the rest of the face's map, it would be okay to ignore the material zone boundary, just you'd need to apply the same map and specularity tricks to the face material as well as the lip.  For other areas that will span a break in the UVMap at a material border, that would be a lot harder (look into a projection painting app).

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