How can I render only bottom layer

[Vasily]

There's 2 layers. 1 (named Model) and 2 (Base). 1 layer looks like a grid (many pipes) which covers layer 2.
I need to render only layer 2, without spending computing powers on rendering layer 1 materials. But it need to be with "blank" transparent spaces where layer 1 covers layer 2.

I want to use layer 1 as mask for layer 2.