Gluing ground cover to foam board

I don’t know if there’s an exact right way to do this. Everyone’s mileage varies on how to secure scenery.  If what your doing is working for you that’s all that matters.

I’ve always put the initial layer of glue on with a brush. But always straight out of the bottle. Not diluted. Once everything is covered. I just spritz it with some water with a drop of dish soap till you start to see the glue come up through the ground. Basically just give everything a damp look. Walk away for a day till it drys.
If I’m still adding fine turf. I just recover the area where needed. I think it’s best to put it on a dry surface. You can sort of see what it looks like and can still move things around or brush them away. Once I’m satisfied with the look. I wet the area again. Then I just use a pipette or mustard bottle and dribble on a 50/50 mix of water and glue. Let that dry.
  If I’m adding ground foam, clusters or anything thick and heavy. I stick with the straight glue and use a small brush in the areas I want covered. Or just dip the piece of turf in dab of glue and place it.

In your case. If your  just adding an extra layer of the same ground covering.  Brushing over the entire surface is fine. You can also just cover it with another layer. Get it damp with some water and then use a sprayer with your 50/50 mix. I’ve used sprayers. Woodland Scenics makes a good one. The only thing I don’t like about them. Is you really need to just apply a mist to give it a good soaking. If you don’t get the glue mixture with the water just right. The nozzle tends to clog on you. I prefer to just dribble the glue on with a mustard bottle or baster.

Trust me. You really can’t mess this up. There’s nothing you do that you can’t go back and fix.

Use undiluted white glue. Spray wet water over the top, then 50/50. Vacuum up the loose stuff. That reptile substrate is going to hold moisture (that’s basically what it’s for) so make sure you get it fully sealed or keep an eye on it and make sure nothing grows.

Generally, I have painted the ground brownish with any cheap house paint I have to create underlying dirt, then sprinkle all the various woodland scenic or scenic express ground covers over the wet paint.   Let it dry overnight.  The next day I usually give it a dry second pass of some various sprinkles of cover, then a spritz with a sprayer of 99% alcohol (but all the wet water type stuff works too), then a decent spritz of a 50/50 glue water solution.   If it's a tight area, I have squeeze bottles (like u used to see for mustard and ketchup at a diner) and use those to do the alcohol or 50/50 solution.  But you have to be careful not to let too much out.  Let that dry another night.  Next day everything is locked in.   The trees, bushes and other larger (non-ground up items) are just glued on with a dab of glue.  Done.    JUST BE SURE to peel off the thin film of clear plastic many of the pink or blue hard foam has on one side!   Not all have it but many do.  Another tip is IF you use the squeeze bottles, you can securely tape up the tip, then punch a tiny hole with something to create a smaller opening than the bottles usually have.   Better control for your application.    

I first seal the surface with a water-based primer   like Acrylic Paint and Latex Paint

several coats ; then I can use oil-base spray paint on the surface.

I brush on undiluted white glue.  Very inexpensive when bought by the gallon.