I have used/use some PVC demolding stuff from my resin supplier to coat the molds with every time I use them, I also use unplanned cutting boards and never considered the planne them. The joints get easy sealed if you give it a few costs on the joints with the PVC demolding liquid, you just have to allow each coat to dry before the next coat is applied.
After that, is only one side that I have to keep sealing and that is the face that I remove to get the casts out. Another thing I found is that, you can use some thick CA from the inside the mold after you put it all together and run a small bead all around, it seals the whole thing completely and when you remove that face (mark it so that you know which one to remove, every time...!) the get the cast out and then you want the use the mold again, after you screw it back into place, a little run on CA to seal that joint again right to the corners, will get you ready in no time. I use the accelerator immediately so that the glue dries on the joint and not run in and possibly out before has the time to set (if there is a gap), it works for me...!
Unless I destroy the molds by melting/burning them, the only time I consider to replace the molds is when it starts to get very difficult to demold, even with the PVC demolding liquid in there. What happens is that, with use, inner the surfaces of the mold get damaged, cut, scrapped, etc, and everything tends to stick to those spots so, it is correct that, these cutting boards have a protective film at their surface, the corrugations are there to prevent sticking, as soon as they is removed/worn out, things are no longer as easy to demold...!
Cheers
George
