Not sure if the title describes it well enough so I took some quick screenshots of basic example. Sorry if it's been asked already, I couldn't find anything in a search.
I have a achieved it....kinda, but it's impossible to retain the same mapping of the dirt because I can't throw a uvw on corona decal.
If I wanted to do a wall with graffiti, then dirt over the top, it would be awesome to have a tick box 'retain layered materials' or something.
Thanks!