Originally Posted by keno
If you are going to try to make cracks, just remember the mortar mix is pretty thick, so you have to over exaggerate the crack. As you are applying the mortar mix you can also use a wooden tongue depressor to get some of the mortar mix out of the crack you created. I have found it a bit tough to make smaller cracks in the faux stones. Maybe before the mortar mix is completely dry you can try scratching it to see if that would work. I plan on trying that technique on my next set of stones.
A note on "etching" the mortar. I pretty much grew up on a construction site, namely my childhood home. The entire inside was cement stucco. My dad didn't do anything little and as such I learned a lot of techniques to add texture, color, and "flair."
In stucco there are traditionally 3 coats of masonry that go over the lath. The bottom coat is scratch coat. Much like the OP's instructions, it is a thin rough coat that allows the heavier second coat to adhere. After the scratch has set, the brown coat is added. This is a much thicker coat. This is where much of the shape is added. In my house, we carved stones, designs, even a dragon and castle, into the brown coat. This can be tricky since it sets up somewhat quickly and carving at it at different stages changes the texture of the design.
For years of experimentation and tool buying the most effective and versatile tools we used were a fork, knife, and spoon. They're riddled with different angles, edges and shaped that lend to pretty much any design you could want to make. I became proficient at the process very young and eventually took over the carving and designs. ( much of this was done on scaffolding at odd angles in high humidity) Anyhow, my experience taught me three things. Go in stages. Do a rough carve or shape on everything first, then do a second refinement, and finally tweak what ever you need to. This allows the texture of each to remain similar.
This project is not a house and is much smaller in scale so much of this can be ignored, I still dabble in playing with cement and I still rely solely on eating utensils. Another trick to adhering the the cement to the foam would simply be elmer's glue. If you do a medium coat over the foam and allow it to begin drying before adding cement, the glue will seep into the cement and the foam. NOTE, if you mix it with the cement it may lend to discoloration.
Because the cement will be sealed with epoxy, you can also use powdered RIT dye to color it. This is a really neat effect for making green, blue, brown or red. Remember your color wheel and go a shade darker than you want. You can add in bits until you achieve the shade you want. One small box was usually enough for a wheel barrow of mud, so a little does go a long way.
Next time I mix up a batch I'll scratch up some samples of some designs. Wow... I got pretty wordy, sorry.