How to mould a motorbike petrol tank

Fibreglass petrol tanks are in a variety of motorcycles, ranging from motocross bikes to custom-built cafe racers. Although a few of these tanks are available on the market, the determined do-it-yourself consumer can build a similar product using off-the-shelf materials. Creating the tank is done by creating a model -- or plug -- from green floral foam, which is then used to create a mould. Although this may be a slow process, using a mould to create the final product will ensure the best results. Expect to spend several days to complete this project.

Creating the plug

Select a large block of green floral foam, or glue several smaller foam blocks together, using spray adhesive.

Sculpt the base of the tank first, carving the foam material with a sharp knife to fit the motorcycle's frame.

Cut large chunks from the top and sides of the tank, using a sharp knife, to create a rough shape of the tank. Concentrate only on cutting the foam to right size and general shape of the design.

Define the shape of the tank, using a sharp knife. Round off or square the edges of the tank and include any details as required by your design.

Stop occasionally to ensure the tank is symmetrical -- evenly shaped on both sides of the tank -- and make adjustments as needed, using a sharp knife to cut excess foam.

Smooth the shape of the tank, using 220-grit sandpaper. Blow sanding dust from the surface of the plug by using compressed air.

Coat the plug with mould-release wax.

Moulding the plug

Prepare your fibreglass cloth and epoxy resin. Cut the fibreglass cloth to fit the shape of the tank, using sharp scissors. Mix the resin in a small container, following all directions provided by the manufacturer.

Flip the plug upside down so that the bottom of the plug faces upward. Apply an even coat of epoxy resin on the bottom of the plug, using a paint brush.

Lay the fibreglass cloth over the resin coat. Spread another coat of epoxy resin over the cloth, using the tip of the paint brush in a dabbing motion. Smooth out any air bubbles in the cloth or resin with a putty knife.

Lay another layer of fibreglass cloth over the previous layer. Coat the cloth with epoxy resin, using a paint brush. Smooth the fibreglass layers, using a putty knife to remove any air bubbles in the resin. Repeat until four layers of fibreglass cloth are placed over the bottom of the plug.

Allow the epoxy resin to cure overnight. Then pull the bottom half of the mould off of the plug.

Mold the upper half of the plug following the process shown above.

Remove any imperfections on the inner faces of the moulds, using 220-grit sandpaper. Fill in low areas on the surface with body filler.

Coat the inner faces of the moulds with mould-release wax and create the final petrol tank, using fibreglass cloth and epoxy resin.


Take your time. Rushing this project will create problems that may not be repairable.


Don't use the polyester resin found at many hardware stores. Polyester resin will dissolve the foam plug unless the plug is covered in foil. Work in a well-ventilated area to reduce the risk of inhaling foam dust and resin fumes.

Things You'll Need

  • Green floral foam
  • Spray adhesive
  • Knife
  • 220-grit sandpaper
  • Air compressor
  • Mould-release wax
  • Fibreglass cloth
  • Epoxy resin
  • Scissors
  • Small container
  • Paintbrush
  • Body filler
Cite this Article A tool to create a citation to reference this article Cite this Article

About the Author

An avid motorcyclist, Chris Gilliland has immersed himself into the two-wheeled world while balancing work life and raising three daughters. When he is not managing the parts department of a local, multi-line motorcycle dealership, Gilliland can often be found riding, writing or working on his motorcycle blog, Wingman's Garage.