If it was possible in Anim8or to apply a textured transparency to a material, over it's ambient/diffuse colour, you'd still have to texture map it to avoid stretching or tiling patterns. This being the case, your best option is to use a bitmap editor (eg. Photoshop) to combine the dirt with the base colour, and then create a series of maps from this image for application in the specular and bump channels to give the dirt dimension and make it less shiny than the base colour, and finally use a UV mapper package to apply it to the object. You'll hit problems if using the ART attributes however, such as attempting to replicate chrome with rusty areas, as the attributes are not affected by maps. In this instance the best option, in my opinion, is using multiple objects. Although I do most of my rendering outside of Anim8or, in part because of the limited scope of its material definition, I would be very interested if anybody else has found a better solution to this problem in Anim8or.
