Applying glue mix to ballast material - Model Railroader Magazine

PED

...I expect to wet the area with a light mist of isopropyl alcohol then apply an adhesive....

As I mentioned previously, regardless of where you're placing the ballast or how thickly or thinly it will be applied, a "light mist" of wetting agent (alcohol or "wet" water) will be insufficient.  If the ballast is not wetted down to whatever it rests upon, the glue will not fully penetrate and your ballast will float when the glue is added.

You can also use the method which Ulrich suggests, but after you apply the full-strength glue and sprinkle on the ballast, the area should have a wetting agent applied, as it will allow the glue to better adhere to the ballast.  That method, with the wetting agent, works for ground foam, real dirt, and pretty-well any kind of ground cover you wish to use, and the method which I outlined in my previous post will also work with ballast, and ground cover of almost any type.

The rock fill (crushed hardened Durabond) and ballast in the photo below, is as deep as it appears.  I applied tape, backed by a board clamped to the layout's fascia, to hold the loose material in place as it was added....

When I sprayed the area with wet water, it penetrated right down to the hardened plaster-on-screen landform, and, as you can see, eventually seeped down behind the tape and onto the fascia, too.  I added diluted white glue with this dropper-bottle...

and continued to apply it until it also began to appear at the bottom of the rock fill.  I then left the area for a few days to allow the glue to harden. 

If I wanted to remove that at a later date, it would be hammer and cold chisel time. 

Don't skimp on the wetting agent and don't skip on the glue!  The wetting agent ensures that the glue goes to where it needs to be, rather than left to harden near the surface where it will be visible.

Wayne