Photoshop uses a project-based painting model which means that 3D objects need to be positioned parallel to the screen so that paint can be properly applied. Setting the Preset mode (from the 3D panel) to Paint Mask will show you the ideal position/resolution that your model should be in for clean paint to be applied: white areas indicate optimal positioning, blue indicates sub-optimal, and red indicates not ideal. The Select Paintable Areas option (in the 3D menu) will select the areas that are in the ideal position for painting.