Wet water is simply water to which a few drops of liquid dish detergent have been added. The detergent lessens the surface tension of the water, allowing it to easily penetrate the ballast or scenery material. As mentioned, some prefer alcohol, although some alcohols react with certain paints. Methyl hydrate in particular is an effective paint stripper for older Athearn, MDC, and Train Miniature rolling stock. It is also poisonous if ingested, and is readily absorbed through the skin, causing nerve and kidney damage. Wear latex or rubber gloves if you're handling it. In my opinion, the detergent works just as well and is much cheaper.
Wet the area to be sceniced thoroughly: it's better too wet than not wet enough. The "wet" water needs to penetrate right down to the roadbed or scenery shell to be effective, as the 50/50 glue mixture, when applied, will only penetrate as far as is pre-wetted. Inadequate wetting will result in a hard crust on top of the material, with the lower stuff neither glued together nor to the scenery base.
To mix the glue, use ordinary white glue, about 50/50 with tap water. Within reason, the ratio isn't crucial: 40/60 or 60/40 works fine too. Some prefer matte medium instead of white glue. It works just as well, but is much more expensive, especially if you buy the white glue by the gallon. Some also claim that the matte medium dries flat, whereas the white glue may be somewhat shiny. The only reason for the white glue to be shiny is because the area wasn't wetted sufficiently before application of the glue. The thicker the ballast or scenic material, the greater the amount of wetting agent and glue mixture required, and this will obviously result in longer drying times.
In this photo, the track ballast and the "rock" fill to the left of the track were all applied at the same time. While there
is a scenery hardshell in place, the riprap was dumped in place along the track until it found its own "angle of repose" (slope), then cinder sub-ballast was dumped along the edge of the roadbed, until it filled in the gaps between the rock. Finally the ballast was added, then the whole area was sprayed with "wet" water until it literally flowed into the field at the extreme left. After that, the glue mixture was added, again until it flowed into the field. I sprinkled some ground foam into the puddles, then sprayed with more "wet" water to induce it to soak up the glue. This took over a week to dry. The scenery to the left was done at a later date.
Wayne