Plaster moulds?

nearly yeah you slap it on by hand but it's not very stable and beggar all will stick to it so if it cracks etc you cant really repair it...ooo and it;s like mdf, it acts like a sponge and soaks up damp so goes all shapes.

believe me it's easier to use grp it really is and it's cheaper too especially as you intend making a grp part from the mould. If mould is to make a one off part just make the mould using 4 layers then add wood to stiffen it.