Wetting with alcohol breaks the surface tension of the glue/water mixture, allowing it to penetrate the ballast. I prefer "wet" water, which is plain tap water with a few drops of liquid dish detergent added: alcohol is for drinking. :thumb: The usual procedure is to spread the ballast mixture "dry", then arrange it with a soft 3/4" brush. When it looks pretty decent, lay the brush handle across the rails, and tap it lightly as you move along the track. This will remove almost all of the stray ballast from the tie-tops. Now, using a sprayer that will allow you to spray a fine mist, thoroughly soak the area with your wetting agent. Apply enough to wet the ballast right down to the base: otherwise you'll end up with a hardened crust on top that's not bonded to the layout. Once the area has been wetted, don't touch it or you
will end-up with a pasty mess. When everything is soaked, apply your 50/50 mixture of white glue and water, using something that will allow you to dispense it in drops. You'll see how readily the glue disperses into the ballast: if it doesn't, or if it beads-up, apply more wetting agent. After you've finished, go find something else to do for a day or two, or longer if the ballasted area is particularily deep.
Here's a photo of an area with deep ballast on top of deep fill. This was all applied at one sitting, and took several days to dry. The ground foam at the base of the rip-rap was added to soak up some of the excess glue that seeped out.
Btw, I'm not a big fan of the foam roadbed: it's easy to use, as long as you don't have a need to taper it down, as for a yard or industrial siding. I'd also be concerned about its longevity. I know of one modeller who had an extemely well-done N scale layout featuring Horseshoe Curve, using foam roadbed. He ended up tearing out the tracks to get rid of the foam, although I don't recall his exact reasons.
Wayne