I'm building this Flying Boat [tail assembly not yet done] and I want the 'Black Rust' texture wrapped completely around the upper wing [each half of the wing is seperate] but I want the bump map for the ribs on the top side only. This seems to call for 'Layers'. [this will be my first] I find the manual's explanation murky...as is so often the case. Can someone kindly state it in simple English?