I'll be milling some moulds out of epoxy. I would love to try the s120 sealer but it's just too expensive to ship to the states. Does anyone know of something similar I can get here? Also I'm curious what type of epoxy/filler will work nicely for this? I will probably be building up a layer of epoxy over pre milled mdf leaving a few mm thickness for epoxy and milling final shape into that. Already tried bondo and that works, but I think epoxy/filler will work nicer and be tougher, just not sure what to use. Another guy mentioned coating the mould in Teflon tape which i might consider, but I want to see what is available to use that is similar to s120. 

Look for mould primers made by whatever company you can get chemical release agents from locally. Not your conventional primers, raher they are commonly a part of a semi-permanent release system so the idea is first you apply the primer, then the sealer, then the release agent. 

Marbocote, Zyvax, Necumer etc. Not sure what you can get in the states but I'm sure someone will have something similar.