How to Fiberglass a Plywood Boat

by Will Charpentier
Fiberglass a plywood boat to add a layer of waterproofing.

Fiberglass a plywood boat to add a layer of waterproofing.

It takes tools and the makings for a fiberglass composite to fiberglass a plywood boat. It involves some elements of carpentry, because you have to be able to measure how much boat you're going to cover. On top of that, you have to figure out how much fiberglass you'll need to cover it, since eight layers of fiberglass cloth and mat only totals 1/4 inch.

1.

Measure the length and width of each area of the boat you plan to fiberglass, with a measuring tape. Multiply the length of each area by its width to determine the square footage of the area. Multiply by eight to determine the number of square feet of 1 1/2 ounce fiberglass mat and the number of square feet of 6 ounce fiberglass cloth required for fiberglass 1/4 inch thick.

2.

Sand the plywood boat with and orbital sander and 80-grit sandpaper. Wipe the sanding dust away with a clean cloth, dampened with mineral spirits. Measure and cut eight pieces of fiberglass cloth and eight pieces of fiberglass mat to fit over each area you wish to cover.

3.

Mix marine epoxy according to the directions on the epoxy. Apply a layer of epoxy to one area of plywood.

4.

Press a layer of fiberglass cloth into the epoxy, using a resin roller. Apply a layer of epoxy over the fiberglass cloth and place a layer of fiberglass mat on the epoxy. Press the mat into the epoxy with the resin roller. Repeat, applying a layer of epoxy and cloth, and epoxy and mat, working the fiberglass into the epoxy each time. Allow the epoxy to cure fully.

5.

Mix and apply epoxy, followed by four plies of fiberglass cloth and mat, following the same procedure used for the last four plies. When the last plies are in place, apply one last coat of epoxy and allow it to cure. Sand it until it's smooth, using a palm sander and 120-grit sandpaper to start with, switching to 230- and 330-grit sandpaper. When the surface is blemish free, switch to 600-grit sandpaper and polish the fiberglass.

Items you will need

  • Measuring tape
  • 1 1/2-ounce fiberglass mat
  • 6-ounce fiberglass cloth
  • Orbital sander
  • Clean cloth
  • Mineral Spirits
  • Marine epoxy
  • Resin roller
  • Palm sander
  • 120-grit sandpaper
  • 230-grit sandpaper
  • 330-grit sandpaper
  • 600-grit sandpaper
  • Long-sleeve shirt
  • Long pants
  • Leather gloves
  • Safety goggles

Tip

  • The finish isn't part of the fiberglass process. The gel coat on shiny, new boats is sprayed into the mold before the fiberglass is laid up, and it won't cure when exposed to air. Since gel coat doesn't do well when applied on top of fiberglass, paint and a good wax job with a wax formulated for fiberglass, is a good choice.

Warning

  • Wear long-sleeve shirts, long pants, leather gloves and safety goggles -- not safety glasses -- when working with fiberglass. As epoxy cures, it gives off heat. If you place more than four coats of fiberglass and epoxy at a time, the heat will cook the resin in the epoxy and the finished product will be plagued with problems like de-lamination, where the layers of fiberglass separate and weaken the finished job.

About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.

Photo Credits

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