Unity "scrape away" effect

I am trying to create a effect similar to what is described here How can I let players "scrape away" one texture to reveal another?

I have tried working through this but am confused and hoping someone could provide more info or if there is another way of doing it. Essentially I need to use a laser effect to "burn" or scrape paint from a door to reveal a layer below. Or any object.

I have a object which I have applied the shader to, I can then apply my "rust" and "clean" texture. When I add the splatmap the "rust" texture shows, when I run the scene.. a mouse click then does nothing.