How to Repair a Styrofoam Hull

A Styrofoam boat hull is a hull constructed of a core of Styrofoam, sandwiched between two layers of fiberglass cloth strengthened with resin. The fiberglass cloth protects the foam, creating a waterproof and paintable surface that helps the foam retain its shape. The combination of the three layers creates a strong and light boat hull, but only if the integrity of the three layers remains intact. Damage to a section of the hull requires replacement of that damaged section with new materials. After replacement though, the hull should work as good as when built, with little evidence of the repair having taken place.

Things You'll Need

  • Utility knife or reciprocal saw
  • Sandpaper, 80-grit, 120-grit and 220-grit
  • Epoxy adhesive
  • Epoxy filler
  • Cup
  • Wooden stir stick
  • Squeegee
  • Marine foam core
  • Plastic sheeting
  • Weight
  • Fiberglass cloth
  • Scissors
  • Orbital sander
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Instructions

    • 1

      Remove the fiberglass cover over the damaged area of the Styrofoam hull by cutting through the fiberglass cloth on the hull with a utility knife or reciprocal saw, then pulling the fiberglass away. Make the cut a square one to ease replacement.

    • 2

      Remove the damaged Styrofoam from the hull down to the interior fiberglass skin. Use utility knife to cut away the damaged portion, and pull it clear until you're left only with the undamaged portion. Keep the section removed as square as possible to ease replacement.

    • 3

      Attach a sheet of medium-grit sandpaper to a sanding block and sand the interior fiberglass hull skin to create a rough texture on which the replacement foam can adhere. Sand the exterior skin with the sandpaper as well, feathering the surface from the inside out so that the edge of the hole begins thin and increases in thickness as it extends away from the hole created by removing the fiberglass.

    • 4

      Measure the empty core space with a tape measure and cut a piece of marine foam core to the dimensions measured using a handsaw.

    • 5

      Mix a small batch of two-part epoxy adhesive using the ratios provided by the manufacturer to combine the two components in a cup with a wooden stir stick. Add epoxy filler to the adhesive until you thicken the mixture to the same consistency of creamy peanut butter. Brush the adhesive over the interior fiberglass hull and across the sides of the undamaged foam in the hole with a squeegee. Place the replacement block of foam into the space then cover the foam with a piece of plastic sheeting and a weight to hold it in place. Allow the epoxy to cure overnight.

    • 6

      Remove the weight and plastic from the top of the foam. Mix a second batch of the epoxy adhesive and filler. Use the filler mix to fill in any gaps between the foam piece and the older foam within the hull. Allow the epoxy to cure overnight.

    • 7

      Cut fiberglass cloth to the size of the hole extending to the feathered edges using sharp scissors. Use multiple pieces to ensure that you can create a skin replacement the same thickness as the original hull skin. Mix the fiberglass adhesive without filler and brush a light layer of the adhesive over the filled foam hole and over the feathered edges of the hole. Pace the first piece of fiberglass cloth over the surface, then brush a layer of the adhesive over the cloth. Build up the cloth layer by layer, brushing each with the epoxy until you have a new skin the same thickness as the old. Allow the epoxy to cure overnight.

    • 8

      Use an orbital sander with an 80-grit pad to sand the skin smooth, removing high spots created with the cloth. Switch to 120-grit and go over the surface again, smoothing it further and smoothing the edges so that they're even with the old skin. Change to a 220-grit pan and complete the smoothing process, removing any seams along the edges so that it runs smoothly into the old fiberglass with no tactile signs of the repair work. Wipe the surface with a tack cloth to remove sanding residue and paint as desired to match the original hull.

    • 9

      Repair damage that does not extend to the foam core by removing the damaged skin and proceeding to re-layer the fiberglass cloth atop the intact core. Sand the replacement cloth as you would for a full breech, then paint the repaired skin as desired.