I am making a mold on my CNC router table in MDF. The image will be reversed
so when I put in the concrete the positive will show when released from the
mold. I am sealing the MDF with spar varnish. My question is.......what can I use as a release agent so the concrete will separate from the mold smoothly. Grease, Pam, silicone spray,WD 40 or whatever? I imagine I'll have to separate them while the mix is still green but set. Any ideas?
These will be used for personalized stepping stones and such.

Thanks