Nitriding general information/discution

Hello

I work in a Open Cavity TPE injection Mold making.. Most of our product are related with car component.

Sometime Loose Piece have to be Nitrided.. Here what i do to prepare a part for Nitriding..

The part come from the CNC.. Make the tool mark go with stone and ceramic stone with the use of a profiler and a tractor..
Most of the time i start with a 220 grid .. than 800 grid.. we dont really go more than that.. no need or mirror finish.. I guess we do A3 or B1 finish ..

With the 800 grid one i use some windshield washer (not the winter one.. but the regular blue one)..

Than i clean the part with Acetone.. And the company with who we deal ask us to mark the section where we want the treatment.. so they can MASK the cavity..

We ask for .001 max treatment

When the part come back.. there is a green powder (the masking).. that is were i get trouble.. once i remove this stuff.. i can see some treatment mark on my part :(
So i got to re-work the polishing of the cavity.. witch have harden some sort... if we dont.. those spot get transfer to the TPE part


What do you use the Nitriding process for ??
What is that green powder ??
What other masking method could we use ??
Is there a better treatment else than nitriding we should try ??