Technique for Preventing Rotten Deck Core

The vast majority of water intrusion into deck core comes from places where there are holes through the outer skin into the core. There is a fairly easy, inexpensive way to prevent water intrusion and the ensuing balsa core deterioration on our favorite boats. Some of the common areas of problems have been the line clutches, handrails, and cleats on the cabin top, and jibtracks, lifeline stanchion bases, bow pulpit and vent holes on the decks. This procedure describes how to prevent water getting to the core if the core is still clean and dry.

  • Drill
  • Assorted drill bits
  • 4" Angle grinder with flexible pad attached
  • Duct tape or Masking tape
  • Mixing buckets and mixing sticks
  • Syringes (West System 807)
  • Epoxy resin and hardener
  • Microfiber filler (West System 403)
  • Your favorite deck hardware sealant (I prefer 3M 4200)

  1. Remove deck hardware where you want to prevent water intrusion
  2. Over-drill the existing hole(s) approximately 1/4" larger than the the existing hole through the top layer of fiberglass and core, but not through the bottom layer of fiberglass
  3. Place a piece duct tape over the inside hole(s)
  4. It is not a bad idea to put masking paper down around your work area - I invitably forget this step and have to clean-up some drip later
  5. Thoroughly mix epoxy resin and hardener in mixing bucket
  6. Add enough microfiber filler to get to the consistency of mayonaisse
  7. Cut the end of the syringe to as large as you can to still get in the hole
  8. Suck the thickened epoxy from the mixing tub into the syringe by placing the cut end of the syringe in the epoxy mixture and drawing back on the plunger
  9. Move to the hole(s) and squirt the epoxy in slowly enough that airbubbles are minimized - It will settle in a couple of minutes
  10. Fill to slightly higher than the surrounding surface
  11. Once the mixture cures (time depends on temperature), grind down the surface level with the surrounding surface
  12. Remove the tape from the inside of the hiole
  13. Redrill the original holes
  14. Use a countersink to make a nice chamfer around the hole - this makes a good place for the sealant to seal the mounting hardware
  15. Use Masking tape to
  16. Remount the hardware using your favorite sealant

Technique for Repairing Rotten Deck Core

You should be able to repair a 12” x 12” piece of deck, e.g. under the chain plates, a notorious place for leaks, for under $200. In the case of this specific repair, most yards use polyester resin for a faster cure time and reduced cost. This is ok since the chain plate area is not a high stress area. However, for maximal strength and adhesion, epoxy resin should be used throughout.

  • Angle grinder with cutting disk and 40 grit sanding disks
  • Chisel
  • Hammer
  • Mixing buckets
  • Brushes


  1. Out with the old: Using the angle grinder with the guard removed and the cutting disk installed, cut through the top layer of glass and the core. Do NOT cut through the bottom layer of glass. Cut a cross-hatch pattern inside the area to be repaired. Chisel out the old core. Make sure that all of the rotten core material has been removed.
  2. Feather the existing deck edge using the grinder with the sanding disk installed.
  3. In with the new: Place a layer of glass cloth/matte, saturated with resin and hardener into the opening. Do not allow to cure. With epoxy use biaxial cloth. Fiberglass cloth works with both epoxy and polyester resins but fiberglass matte is only suitable for use with polyester resins.
  4. Place a piece core material cut to size on top of the base layer.
  5. Follow with layers of glass cloth/matte saturated in resin and hardener to almost fill the opening. This will take multiple layers. Allow the intermediate layers time to set.
  6. Finish filling the hole with fairing filler. When set grind/sand to a smooth finish.
  7. Prime and then finish with the non-skid paint.

See Also:

  • Independence
  • Nut Case
  • Taz