Lerping certain sections of a roughness map?

Development Rendering
1

Hi guys! I am wondering how to setup a material for my tommygun so that it allows me to Lerp, two different sections of a roughness map.

I am creating a Thompson gun and want to alter the black and white values of the metal and wood seperately. How would I go about doing this?

My current Gun - http://i.imgur.com/CeauSfT.png

My current material setup - http://i.imgur.com/2e83vaX.png

The node which I have selected is a mergemap which is a mask for the metal and the wood.

2

Since you already have a Metallic mask (the texture plugged into the Metallic input, why also have the mergemap?

You could do something like this:

3

Thanks for the reply pylb. I took what you said and used this node network(http://i.imgur.com/fJ97qpN.png) to get the desired result.