Rotten to the Core: Fiberglass Core Repair, Part Two

In my last post, I explained how to identify where the core of a fiberglass boat needs repair. When repairing core problems it is important to find the extent of the damage.  Moisture meters can help, but I find simply drilling test holes into the suspect area more effective. Drilling will definitely show you if water has gotten in, and if you see rot on the tip of the drill, carefully drilling more holes will show you the extent of the damage. Using a marker, note which test holes contain problems.

Exposed Rotten core with test holes showing the extent of the damage

You’ll need to keep the area dry while you repair it, so getting the boat, or at minimum the area of repair, under cover is helpful. The next part of the repair is to remove as much water as possible, so go ahead, drill some holes on the bottom side of the affected area and let it drain.  You will have to open up one side of the fiberglass sandwich skin to get at the affected area.  Before you cut the skin back to remove the rotted or delaminated core and allow access to lay in the fresh material, connect the test holes with a marker to show the extent of the damaged area and act as a guide for your cut. I usually like to remove the top skin and let gravity help with the repair.  In other words, if you have a delaminated deck, repair it from the top. If the problem is in the hull, repair it from inside the boat using the outside skin to support the repair. There are some exceptions to this, particularly if you have a molded surface such as a hatch opening or non-skid that would be hard to replicate. In that case you may choose to go in from the underside. The best tool for removing one skin area is a roto-zip set to a shallow depth.

Repairing the deck of this boat was done from inside the cabin because of the difficulties replicating non-skid. Notice the ingenious way that the repair is supported by spring loaded shower rods.

Carefully pry off the skin and use a sharp chisel to remove rotten or delaminated core.  You’ll want to keep going until you are into solid core.  If the area is still wet, allow to dry. Acetone can help drive moisture out in a pinch, but remember to wear latex gloves.  Now cut and dry fit the replacement core.

Tape up any drilled holes with masking tape.  This will keep the liquid epoxy from oozing out as you apply it.  You are now ready to epoxy in the new core.

Before you epoxy here are a few tips for staging your materials, and techniques for epoxying that will help make this go smoothly:

Have an adequate supply of latex gloves, disposable quart containers, foam brushes, stirrers and acetone.  Chandleries such as West Marine or online suppliers such as McMaster-Carr carry most of the materials you will need for the repair. Core material comes in various thicknesses. You can also obtain it in rigid sheets or sliced and mounted to backing paper(preferred) for use on contoured areas. Note: epoxy gets hot when it cures so have a clean trash barrel for waste and don’t mix acetone laden rags in with the refuse to avoid a fire.  Epoxy will set up in roughly ten to fifteen minutes depending on air temperature and which hardener you choose, so you’ll have to work fairly quickly.

I find the quart containers the right size for mixing and applying the goo before it starts to “kick”.  I prefer West System Epoxy from Gougeon Brothers because it will bond to almost any prepared surface—fiberglass, wood, foam…  If you buy all the measuring pumps and follow the instructions it is pretty straightforward. When doing this kind of repair with West System, I recommend using their 403 Microfiber Adhesive Filler for a better bond.

The thing about West System Epoxy is you can keep adding resin while it is “green” or not completely hard, but upon hardening it emits a surface sheen called Amine Blushe, which needs to be sanded between coats to form a mechanical bond.  In a perfect scenario you want to avoid sanding if possible and take advantage of the “green” chemical bond– even if this means making up several small consecutive batches of epoxy resin.  Simply throw out the old container and applicator, change gloves and keep going.

The repaired area awaits painting and the remounting of hardware.

Using a disposable foam brush, generously “wet out” the inner skin with epoxy mixture, do the same to one side of the core material, and lay the two wetted surfaces together.  Use peel ply or wax paper to cover the area and then put something heavy on top of the wax paper to hold it in place until the epoxy is completely set.  For larger repairs you may want to do a section at a time. With the core in place, take a piece of cardboard, cover it with wax paper and fiberglass roving, wet out the top of the core and the roving,  and secure in place until set.

When remounting any hardware, overdrill the holes, tape and re-fill with epoxy. In this way, when you drill the right size hole through solid epoxy, you will have sealed the core against further water intrusion and a repeat of the problem.

For a couple of hundred dollars in materials and plenty of patience, you can make an old boat new again.

Editor’s Note: This is part two of a series on fiberglass repair.  Read Repairing Fiberglass Boats, Part I


f you’re getting your boat ready to sell, a moderate amount of time and effort will add genuine value and help justify your asking price.
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