Is there an equivalent to Gelcoat for casting plaster?

Question

I have experience casting fibreglass canoes. When I cast a canoe I first coat the inside of the mold with Gelcoat for a smooth, fibre-free exterior.

I am planning on casting some little plaster/resin trinkets, that I will mold in silicone. I would like to reinforce these with fibreglass or copper wire.

How can I coat the inside of the mold so that I have a smooth, fibre-free exterior? Is there anything else I have overlooked?

Answer 1

Gelcoating the mold works with fiberglass because the resin bonds with the gelcoat. Plaster won't (the gelcoat layer will just be a smaller mold). There are a few options.

• Reinforcing material: Fiberglass cloth won't be a good material for plaster because the plaster won't penetrate the cloth like resin does. You could probably use fiberglass wool, but coarse, open-weave fabric, like burlap or gauze, or drywall repair mesh, will work better and is easier to use. With plaster, you will avoid "wild" fibers showing on the surface if you first put down a surface coating of plaster, then add plaster-soaked fabric or embed mesh in an additional plaster layer.
• Surface coating: If the gelcoat is also to provide a glossy resin surface, you can accomplish that after the plaster has fully hardened (many days so there is low residual moisture in the plaster). You can brush, spray, or dip the finished piece in any suitable finish.
• Casting material: If a matte, resin-looking surface will work, you can use a "polymer-modified" plaster and avoid the need for coating it. There are a few off-the-shelf casting materials, like Jesmonite, that mix plaster with acrylic polymer liquid instead of water. The result is much stronger than plaster, and looks like a resin casting (although matte instead of glossy).