Plaster moulds?

[ayjay]

I,ve done a bit of moulding and the advice so far is generally good.

However if you need to design a body panel from scratch then plaster is a good idea. It willneed a fair bit of sanding ,scraping ,shaping smoothing (all the s's eh!) in order to make a model .You could then use that to make your pattern mould out of GRP.
You would save a lot of time by making a rough shape out of polystyrene blocks but slightly smaller than needed then smooth on your plaster . then the hard work comes to refine the shape as S's above.
Dont use pink plaster the stuff called "onecoat" is a lot more forgiving and easier to handle( white in colour --Wickes etc)
You can easily stop it cracking by reinforcing it with thin bandage and shave it down with an angle grinder(not indoors) .Get the final mould surface with something like car body filler fine sanded to the final shape.
We used to use silicone spray as a release agent but theres probably something better specially for releasing GRP.Hope this helps---ENJOY