How to Build a Fiberglass Box

How to Build a Fiberglass Box

Building a fiberglass box yourself ensures that you have the right sized box you need. It can be hard to locate the right sized fiberglass box. So, you should consider doing it yourself. It’ll take some patience when it comes to drying the fiberglass box. However, the yield is great. Here is how you can make your own in a moderately easy do-it-yourself project.

Required Materials

  • foam
  • Bondo
  • fiberglass mat and strips
  • paint

  • resin
  • resin colorant
  • sandpaper 400-4000 grit
  • polyethylene two-part construction grade form

Required Tools

  • orbit sander
  • serrated kitchen knife
  • handfuls of brushes
  • mixing sticks and mixing dish
  • small chisel
  • measuring tape
  • pencil/paper


Step 1

Determine the purpose of your box. For example, if your fiberglass box will be used as an insert for another structure, measure the space or structure first. Draw a plan with the correct dimensions stipulated so you can make sure the fiberglass box will be a perfect fit.

Step 2

Make the mold for your fiberglass box by cutting the right shape out using construction foam.  Make sure the edges are straight. This is a very important step. Fiberglass won’t usually form an exact 90 degree corner. After the foam box is made, paint the surface. Apply three coats of paint. The paint is a barrier between the surface and Bondo. Use the Bondo to finish the shape into exact measurements.

Step 3

Allow the Bondo to dry but not to a hard finish. File the piece as closest to the actual size as possible. Make sure the surfaces are fairly smooth. Allow to dry to a hard finish. Paint the surface. The paint will highlight the flaws on the surface. Sand them away.

Step 4

Apply the fiberglass by coating it with resin. Lay it over the mold. Keep wrinkles and imperfections to a minimum.

Step 5

Sanding the surface where pieces come together. Begin with a 400 grit piece if sand paper or your orbit sander. Any other sander type with badly scratch the fiberglass. In some instances, the waste can leave a good-sized pit in the finish. Advance 4000 grit sanding materials. At level 2000 grit, you should be able to see yourself in the finish.

Step 6

Remove the mold carefully and gently. If it is difficult to remove, gently chip away the foam and Bondo until it you can remove it completely.

Step 7

Paint the box using a paint gun. If you desire a piano finish then only use resin. For an opaque finish, mix the paint with a color.

Step 8

Enjoy your new fiberglass box.

Filed Under: Home & Maintenance


About the Author: Jason Prickett loves to write about home maintenance and stuff you can do yourself instead of hiring any professional. His step by step guides will assist you in completing your home maintenance tasks.

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