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I don't know about Silo, but in Anim8or the shell tool does just what you described in your first post - duplicates the surface, and joins the two surfaces around the edges. It's much easier that way, and you have interactive control over the thickness of the result. Just drag the mouse further to increase the distance. I find it's a good idea to bevel the edges after "shelling" to smooth the result.

Yep, that's exactly what the Silo "shell" does.

So, to replicate what I thought rjjack was doing:

• Model dress
• Use Shell to give it thickness
• Select internal polygons and set their normals to point away from V4
• Assign seperate materials to inner polygons, external polygons and the "thickness" strip of polys that links the two

 At least, that's how I read it, but I've been wrong before :)