How to get a Fresnel node to only affect one part of an object? Ie) The top of an object Just had a quick question.  In the image below, I am trying to get dust to show up at only the white parts of an image.  I am trying to get a PBR dust effect where when you look at it, at an angle, you can see the dust film better.  I know you do this with a fresnel node.  The trouble that I am running into is that there is fresnel on the whole object and where white is on the object, that's where dust is supposed to be.  I was able to access just the top parts of the model where dust should physically lie (second image below) but I can't figure out how to get the fresnel effect to work only on this top data.

The problem where I am getting fresnel everywhere:


The top facing values where I only want the fresnel to work on (I want it to work on the white values):


What're your guys' thoughts?
You are currently blending the fresnel and topside(normal) masks using a mixRGB node set to "mix". This gives you a blend of both masks. Sounds like what you're actually after is the area where both masks are non-zero, so the blending option you want is "multiply". You can either change the mode on the mixRGB node to "multiply", or replace it with a math node since these are float values (grey socket) rather than vectors/colors.