Fake Fiberglas rocks and driftwood

[oceansunltd]

My friend Peter Brady at New England aquarium builds all the interiors of their exhibits. Some rocks and wood is real, but a LOT is fake. Here is how he does it.
First, find the surfaces you want to copy. For rock work, you want to to find a surface with no "undercuts" that will "trap" your mold. Spray with silicone "mold release agent" or even light cooking oil. Buy some liquid latex at a craft shop. VERY, VERY carefully paint on a thin even layer and let dry until tacky. Paint on a couple more layers, then imbed a layer(or two) of cheesecloth onto the wet latex. A fianl coat over the cheesecloth. Let cure for a day or so. CAREFULLY peel away from the rock surface. You now have what is called a "negative" mold. This can be used in two ways. First, you can "build" a similar size "rock" out of styrofoam block, shaping it with hacksaw blades and rough files. Cover the styro with a couple coats of cheap latex paint (this will keep the resin from melting it). Paint on a coat of resin (polyester works fine) on top of the latex coated styro, then a layer of cloth, then more resin. A single layer of cloth is enough if you're not going to be stepping on it-if you are, at least 2 layers thick. Keep working while everything is wet with resin. When your glass layer is thick enough, lay on a coat of resin thickened with either fumed silica or phenolic microballoons. Your local boat store carries this stuff. This coat must be thick enough to receive the detail from your mold, but not so stiff it can't take the impressions. Lay the mold on top of the resin and press gently so that it comes in complete contact. LEAVE IT ALONE OVERNIGHT. Peel away the mold, paint with appropriate colors, and you have a fake rock. Dissolve away the styro form with gasoline or acetone, and intakes may be hidden beneath your now hollow rocks.

[oceansunltd]

The second way to use your negative mold will provide greater detail, but it's real tricky to do on good size pieces. Suspend the mold upside down, and either spray or paint a LIGHT coat of resin onto the inside of the latex mold. Let cure. GRADUALLY build up the layers until it's stiff enough to lay resin wetted glass cloth onto it. Let cure hard, peel away the mold. You'll have an EXACT duplicate of what you originally cast. Paint it up and you're done. The tricky part is the fact that the mold has no structure to hold it's shape. We'll often use damp sand under the mold to provide initial support, working small areas of the mold at a time and letting cure until we have a shell. Then it's easy.