Also, in relation to the issue of the rust changing colour it is possible to fix this. Create a colourpalettemap for the body colour and set the rust up as a third diffuse map. You can also adjust the reflectivity of the rust by adjusting some values in the materials.cs for a more realistic effect. AFAIK this is done in the vanilla rusty skins (although I haven't checked myself). Maybe unpack those and see how they are done.