Fiber glassing a large positive mold.

[Hawaii00000]

If I was to fiber glass a large positive mold like this one http://blog.makezine.com/archive/2007/03/how_to_large_scale_mold_m.html , how could I keep the fiberglass from sticking to the plastic? People seem to wax the surface a lot but its hard to wax something like thin plastic. I'm thinking maybe petroleum jelly.

[paulstreats]

could you warm the wax up?

the trouble with petroleum jelly is that being slightly fuidic the fibreglass might try mixing with it or might even just push through it and stick to the plastic anyways. wax is solid so it cant push through it. I'd warms the wax to a liquid, brush it over and leave it to solidify.

[Soeren]

Hi,

If you want to make the finished piece directly on the positive mold (without the intermediate negative), I see no problem in letting the plastic stick, as you can just sand it off afterward if it bothers you.
Not using a negative mold means a lot more finishing work with sanding etc. with perhaps a top layer of GelCoat. Using a negative mold, you start off with a couple of layers of GelCoat before using glass cloth at all and if you had a smooth positive to make the negative from, you will have a just as smooth finish.