Hi Vinced

The "dirt" at the edges is caused by an "anything goos" plugin within the shader tree.
I don't understand the term AO-Shader?

I normally do not use any GI rendering and NEVER get involved with UVs.
Attached is an example of the "rusty edges" shader tree set up.
Carrara8 Pro.

Brian
ps I just remembered about the built in AO option in Carrara--which I have only occasionally used.
I tend to render at the default render settings--because they are fast!---then take into Photoshop and, usually, just adjust levels. Much faster.