How to keep Structolite from cracking as it dries
The Structolite is drying out too fast, because the bare brick is wicking out all of the moisture.

You need to use a bonding agent and possibly add some sand to your base coat, as well as scarify it, for any successive coats.

If this project is going to have any substantial thickness, it probably wouldn't hurt to also add some kind of structural support, like wood and/or metal lathe.

I use the latter all of the time, when doing repairs where the plaster has fallen completely off sections of wood lathe.