I switched from white glue to matte medium several years ago and prefer it. I use artist matte medium that I buy in a craft store (it is not a concentrate). I find that a mixture of 50% matte medium and 50% water works well. I also add a few drops of dish detergent to break the surface tension so that in flows on easily. A higher percentage of water also works (60% or 70%), so the 50/50 mixing doesn't need to be exact. Because I add detergent, I don't pre-moisten the area to be scenicked.
Occasionally I do get the problem with white residue. It is due to using too much matte medium mixture. When it can't adsorb down into the scenic material and stands in pools on the surface, those areas will sometimes have the white residue.
