Have the model shave off any excess body hair from the portion being molded. A torso cast typically extends from the neck to either the waist or mid-thighs, excluding the arms.
Have the model dress in a tight pair of underwear if molding to the thighs. Otherwise, an old pair of jeans will suffice. Tuck a trash bag into the jeans like a bib to help keep them clean.
Brush a thin coat of petroleum jelly over all exposed skin (and underwear if applicable) to serve as a mold release.
Mix a large batch of algenate and cold water, following the mixing instructions that are included with the algenate. Algenate does not stick to itself when it sets, so you must mold the entire torso in one shot. The cold water will retard the algenate's setting time, giving you a few extra minutes.
Have the model hold both arms straight out like an airplane. They will not be molded, and are thus kept out of the way.
Smear the algenate over the front half of the model's torso. Two to three people are recommended for this to get the algenate in place quickly. If it drips, scoop it back in place.
Continue scooping the algenate in place until it gels, turning from a paste into a rubbery solid. This typically takes five to eight minutes.
Cover the gelled algenate with five layers of plaster bandages. These should be cut into 12-inch strips, then dipped in water and smoothed onto the algenate. The bandages will form a rigid support shell, helping the flexible algenate to keep its shape.
Allow the shell to dry and harden. This will take 10-15 minutes.
Have all workers place their hands on the mold, then have the model lean forward. A bit of wiggling will pop the mold free.
Tear a large amount of fiberglass mat into 5-inch squares.
Brush acetone into the algenate mold to dry the surface slightly, then spray two thin coats of Crystal Clear into it. Allow each coat to dry before moving on.
Pour some auto body filler into a disposable bucket, then thin it to a brushable consistency with fiberglass resin.
Mix in the catalyst for the filler and resin, following the mixing ratios provided on the instructions.
Brush this catalyzed mixture into the mold, covering the entire inner surface. This layer will capture the mold's detail, and should be one-fourth-inch thick.
Allow the layer to harden. This may take several hours depending on the moisture content of the algenate.
Mix a batch of fiberglass resin with catalyst in a fresh bucket.
Brush catalyzed resin onto the dried filler layer.
Lay squares of fiberglass into the resin, overlapping them slightly to ensure that there are no gaps.
Brush more resin onto the fiberglass, saturating it fully and working out any air bubbles with the bristles of the brush.
Apply four more layers of fiberglass and resin.
Allow the resin to cure for several hours until it has completely hardened.
Peel the algenate mold off of the cast. This will destroy the mold: throw it away.
Cut any rough spots off of the edges of the cast.
Paint and display the cast as desired.