How to Make a Fish Cast From Plaster of Paris or Fiberglass

Updated April 17, 2017

You can make a replica of your prize fish by using plaster of Paris to capture its details. With a clay mould, the image of your fish will be incised for plaster casting. Plaster of Paris has been used for thousands of years. When water is added it undergoes a chemical reaction and becomes hard and capable of picking up details from a mould. Fibreglass and polyester resins may also be cast into a clay fish mould.

Roll the clay into a 3-inch slab with a rolling pin; make it long and wide enough to accommodate your fish shape plus two inches of border all the way around the fish. Roll the clay on top of an old sheet; this will help get the clay off the table easily as you can simply peel the fabric off the clay.

Place the slab onto the plywood. The wood will hold the clay securely and keep it from bending.

Roll over the top of the clay surface to make a smooth top.

Press the fish firmly into the clay.

Push the fish down into the clay until half of it is embedded in the clay, lengthwise. Pay close attention to the fins and make certain they have enough pressure to leave an impression in the clay.

Remove the fish carefully from the clay. You will now see the transferred image of the fish in the clay, and you have a working mould.

Mix the plaster according to the manufacturer's instructions. Always add the plaster to the water. Sprinkle the plaster evenly and slowly into the water until a small conical island appears in the centre of the water.

Allow the plaster to sit for five minutes.

Mix slowly with a paint stirrer until the mixture is the consistency of double cream.

Pour the plaster into the mould slowly until the fish indentation is completely filled.

Tap the mould gently to release any possible air bubbles.

Allow the plaster to dry overnight. It will feel hot soon after pouring as a chemical reaction is occurring. It will seem dry to the touch in 30 minutes.

Remove the plaster fish replica from the mould.

Create the clay mould as in Sections 1 and 2.

Layer polyester resin with fibreglass to fill the fish form.

Allow to dry overnight.

Lift and remove carefully from the mould. Be especially careful around the fins.


Pour the plaster into the clay mould while it is damp. Plaster can be reinforced for strength using wire, wool or straw. To make a three-dimensional fish, repeat the steps and glue the two halves together. Plaster can be painted when dry.


Wear a mask when pouring plaster as the small gypsum particles can get into your lungs. Plaster, both dry and wet, can clog plumbing. Do not put it down the drain.

Things You'll Need

  • Clay
  • Old sheet
  • Rolling pin
  • Plywood of a size to accommodate your fish size
  • Plastic bucket large enough to mix the desired amount of plaster
  • Plaster of Paris
  • Paint stirrer
  • Fibreglass and polyester resin
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About the Author

Patti Perry is currently attending West Virginia University and expanding her knowledge base. She has worked as a freelance visual artist for 30 years, with specialties in watercolor and scherenschnitte. Originality of creation is her motivation and she continues to pursue this avenue in her writing. Perry is currently contributing articles to eHow.