Here are some fake rock lessons I've learned. If you're going to create a fake rock project with the idea of a semi-jigsaw puzzle in mind, there's the problem of the pieces gradually sticking together -mainly because of the sealant that seems to dry rather tacky. Sometimes I will pull apart two mountain pieces from a project and little remnants from one piece will be stuck to the other piece.
The solution is to sprinkle some fine grain sand (smaller grain) onto the sides or sections whereby the two jigsaw pieces meet. This smaller grain sand is tiny enough not to disrupt the "fit" of the two jigsaw pieces, and what it does is create a dry layer whereby it's virtually impossible for those two pieces to stick together.
Here's another thing you don't think about until you have already done it. After you apply the non-toxic sealant you're going to use, the base of the fake rock project tends to stick onto whatever material it's sitting.
If you can find something like a metal grate or anything metal-based you'll not have pieces of plastic bag or the tarp you may
have used on which to do the project.