In a previous article I talked about using Titebond II glue to hold ballast in place on your outdoor layout.
In the years that my layout has been in existence, I have found that the rain, puddling around the track,
causes the glue to go back to a liquid and the ballast to wash away.  While Titebond II is waterproof, it will not resist continuous submersion.  During our storm season last year, I had portions of my layout that stayed wet for a long period of time, and consequently I lost some ballast in those areas.
    This spring while re-ballasting my track, I decided to try a cement product to bond my ballast.  Home Depot sells a product called Fast Setting Cement Patcher by SAKRETE.  I mix this with my ballast in the amount of 2/3 ballast to 1/3 cement.  I spread this ballast mixture along the track, I then use a spray bottle of water (with a few drops of dish detergent for a wetting agent) to heavily wet the new ballast.  The cement sets up in about ten minutes and holds the ballast in firmly in place.  When this mixture has dried for about one hour, it will resist any rain caused ballast washout problems.