How to Build a Fibreglass Box

Fibreglass is an excellent material to use when replicating the shapes of specific items. Sometimes the exact size and colour in a box are not available, and making your own is the only reasonable solution–using fibreglass is the best material suited for projects of this nature. The project is simple but will take a bit of time. The results, however, will yield a good-looking product that satisfies your requirements accordingly.


  • Foam
  • Bondo
  • Fibreglass mat and strips
  • Resin
  • Paint
  • Resin colourant
  • 400-4000 grit sandpaper
  • Polyethylene two-part construction grade form


  • Pencil
  • Paper
  • Serrated kitchen knife
  • Painting brushes
  • Small chisel
  • Measuring tape
  • Mixing dish
  • Mixing sticks
  • Orbit sander


  1. Acquire the dimensions for the space where you will be inserting the fibreglass box. On a piece of paper, draw a design for the box including its dimensions. If you are unable to draw the design yourself, enlist the help of someone with drawing skills to get it done for you.
  2. Carve the shape you want to construct in foam; ensure the edges are straight and at perfect angles. This aspect of the construction should not be overlooked–no matter how careful you are, fibreglass will seldom form a perfect 90-degree corner.
  3. Coat the basic foam shape with paint several times; it will act as a barrier and react well with the Bondo.
  4. Finish the shape with thick layers of Bondo. Once the Bondo forms the desired shape, allow it to dry but not to a hard finish. File down the foam Bondo structure to get it close to the intended project size, making sure that it is fairly smooth.
  5. Now allow the Bondo to dry to a hard finish and then paint it with any type of paint; let that layer dry. Take note of any imperfections and sand them down to the correct proportions.
  6. Apply the fibreglass by first coating it with resin and then laying it over the mould. For the corners and smaller pieces use thin strips of fibreglass. For the larger areas, use sheets of fibreglass to cover it. While laying the fibreglass, try to keep wrinkles, pits and defects to a minimum. Keep in mind that if imperfections are left on the project they will spread throughout and give it a bad appearance. Sanding will also be difficult later on.
  7. After the fibreglass is left to cure overnight, start the sanding. Sanding is the most important part of making the fibreglass box and should not, to any degree be neglected. Start with a 400-grit paper on the orbit sander. As you sand ensure to keep the project clean of the sanded fibreglass waste. If the waste is not brushed off, it can leave noticeable pits in the finish.
  8. Sand the structure, working from 400 grit to 4000 grit sanding material. At the level of 2000 grit, you should be able to clearly see your reflection in the finish.
  9. After the sanding is completed satisfactorily, remove the mould. If it poses difficulty when coming out, chip away at the foam and Bondo until it can be removed.
  10. Paint the end product to your desired finish, ensuring it is smooth. Allow it to dry completely.
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