I should probably include an example of what you can do with the simple shader I've included, so...

Each shield in this image is the same model with different overlay masks (i.e. shield designs), different diffuse/specular colours for all three layers, and the same wear pattern mask.

I'm quite pleased with how this is going - once I get the reflection and bump added to the shaders I think I'll have reached my goal for this little project.

(I've already released a prototype of the DAZ Studio version of the full shader as the 'SDMS Shader (beta)' freebie, and I've got a more-or-less working Poser version too - some question threads about this will be appearing soon)

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The 3Dcheapskate* occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.

*also available in ShareCG, DAZ, and HiveWire3D flavours (the DeviantArt and CGBytes flavour have been discontinued).