Thanks HT, will try that out.
Easyscor, in this context "baking" means painting an effect onto the main skin that could also be done with one or more of the effects maps.
So for example it is possible to create an illusion of the metal areas being more shiny than the painted areas of a skin on the base skin itself. To do this I paint bright white highlights on appropriate areas of the skin like the upper half of the fuselage, along the highest part of the wing etc. The layers for these highlights are then placed below the painted markings layers, but above the aluminium layer, so the highlights are not seen on the painted areas. Now this "baked-in" effect can look good when the plane is facing the right way up but odd when it is upside-down since the highlights look like they are reflecting light from below. The specular map however creates shiny areas whose highlights are based on the position of the sun so does not have this problem.