2nd try on ground cover question

I think you're looking to use a 50/50 mixture of Titebond II and water.

-Brian

You're probably thinking of "wet water." It usually consists of a 50/50 mixture of glue (Titebond II for outdoor use, white for indoor) and water, plus a little dishwashing soap to help reduce surface tension. I've also seen matte medium used in place of the glue, but I don't know how it would hold up outdoors.

That's what I've used. Good advice!

Are you thinking of water activated dry resin glue? It is made from animal by products, like horses hooves. I know when I was in HO that is what was recommended to put down ballast with. It worked well inside, but I don't know about outside. I assume that you won't be leaving your building out in the weather, so this might work too. I don't even know where you would get it, as I haven't used it in about 100 years. Actually, I think the Titebond would work better. Mix 50-50 with water, add a few drops of liquid dish detergent and spray away. Try to do it in layers so you get all of the dirt wet. Good luck with your project.