My method has been to spray a light mixture of yellow carpenter's glue, water, and two drops of dish detergent. The only time I use alcohol is to pre-wet ballast, but I use the same mixture of glue for the ballast. For one thing, a 5/1 mixture of water to glue will still dry hard. It is easily sprayed, and it does a good job of holding the ballast in a monolithic piece without making it a solid lump. Later, if you must to a track repair, you can easily bust it up with a metal tool, or you can soak it for five minutes and it will fall apart.
Others prefer to dilute something like Mod Podge or any other matte (not glossy!) medium. In that case, probably something like 3/1 or 4/1 in favour of the water.