An acid etched or phosphate finish, applied typically to shotgun actions, forming a gray-colored, non-relective matte finish which also provides some protection from rust. Allso called, gray-etched.
You will have to describe what you are going to do. As one guy said once upon a time... "There are as many different ways to do French grey as stars in the sky."
I always follow the theory of KISS, Keep It Simple Sandy. I simply say that it is a pickling process that turns the metal gray, that helps impede rust formation. It also provides a contrast of color on a firearm. When I am asked by a client how I do it, I always leave a little mystery in the mix. I say it is an acid solution that the metal is bathed in. This usually does the trick.
Doing a professional job is not in what you use but knowing how to use it.
Sandy