Hi Philipp,
The problem with using a model board for a casting mould is getting the dimensioning perfect for applying a sealing coat of resin over the board itself. Even with very high density boards, we would not recommend using the raw board as a mould without some kind of surface sealant to give a smooth surface. Yes, with multiple layers of mould release agents, you can often get an ok release off high density model boards, but the mould life is short.
When sealing the surface, if you use a resin, you really need to try and take into account how thick your resin will lay. The nature of coating resins onto a surface means it is hard to retain dimensional accuracy and high levels of detail. Thinner coatings such as paint can solve the dimensional accuracy issue, however many paints will struggle to adhere perfectly and be tough enough for the rigors of use as a mould.
So taking this into account, if the loss of accuracy is ok, then you would be fine to use a coating resin such as our
XCR Epoxy Coating Resin. It is easier to sand and shape than a typical epoxy resin, yet not as thick as a tooling gel-coat so will not loose too much accuracy.
In terms of release agents, I would almost certainly recommend using a modern chemical release agent such as our
Easy-Lease Chemical Release Agent.
Warren PenalverEasy Composites / Carbon Mods - Technical Support Assistant