Here's my experience working with Castin' Craft, just for your info. I used it many, many, MANY times and the backs were always tacky. The only thing that helped was using the Castin' Craft brand surface hardener and doing two pours. So the initial pour, wait for it to harden, then do a thin second pour mixed with the surface hardener. The only bad thing about the hardener is that it gives the resin a slightly cloudy appearance. If you are mixing colors with the resin, it's not noticeable, but definitely is if you want it clear.
Other than that, gluing a small piece of felt of the back of my pieces was my only other solution.
I use Easy Cast brand resin now and love it, I would never use the Castin Craft again. I can work with it much longer before it starts to get tacky and the surfaces and backs are always very smooth and non-tacky.
Best of luck!