Best Way to Fasten Cast Rocks to Hard Shell

I use Liquid Nail Project glue.  It holds nearly everything to everything well enough.

TRY USING JOINT COMPOUND. BUTTER THE BACK OF THE ROCK AND SLAP IT IT PLACE

We used hot glue for a lot of our plaster rock molds, as well as the foam rocks. You can use plaster, sculptamold, liquid nails the project stuff works well, Plaster of Paris... pretty much anything. Just wet the surface 1st, if using another plaster, to get a good bond without sucking all the moisture oit of existing work.

I'm with Lee -- Liquid Nails.  I've had good results with the panel and foam adhesive.

 

Poppyl

Thick hydrocal applied to the back works.

     xrayvis if you are going to use plaster, sculptamold or hydracal make sure you wet the back of your rocks before you (butter) them so the rocks don't draw the moisture out of your (butter) and dry it out prematurely. Choo Choo Kenny

This rock detail was done with construction adhesive, attached molds to wood backing.

 

 

My adhesive of choice is Loctite Power grab an instant grab construction adhesive.  A couple of dabs on the back and it will take hold and stick to just about anything porous. I've used it on numerous projects and its always done the job.

I'll just report, since I'm the O.P., that I mixed Structolite in a 3:1 ratio to water (instead of the normal 2:1) and used that as my "mortar" for the cast rocks. I saw a YouTube video where someone did that. Consistency was similar to peanut butter. I wet the Structolite hard shell and the backs of the rocks and everything stuck.

 

The reason I decided to use the Structolite was mainly because I have lots of it left over. Granted it was a lot messier than using Loctite or Liquid Nails for Projects but it had an added benefit of allowing me to fill in the openings and crevices between the cast rocks and the hard-shell. Shockingly, since I have no artistic ability, everything turned out really nice.