I don't know how picky you are about your rocks but once you get Great Stuff
on them it is almost impossible to get off. I decided I didn't like it and
it didn't solve my problem. That isn't to say it won't work. Just that you
might try it out in a 'test' area to get the feel of it and all. It expands
really quickly and is difficult to control. It sticks to anything and
everything.

I'm not sure what Great Stuff is made of but if it's polyurethane there
are cans of low and high expansion PU available at hardware stores.

Does it say anything informative on the label?