Rot Repair in Fiberglass Boats

Wood-Cored Decks
Rot and general wood deterioration in cored decks is a particularly difficult problem, largely because it's so hard to get at. We hear more about it in sailboats than power boats, probably because sailboats have more hardware sticking through their decks (and therefore more locations for the water to get in), and because they are often driven harder in worse weather conditions than power boats. Below is a general diagram section of a sailboat showing the cored decks and cabin top. What this doesn't show are the sail tracks, chain plates, stanchion bases, winches, cleats and so forth that are poorly bedded or work loose and allow the water into the wood interior. Believe me, the water is bound to get in. The wise skipper will apply CPES™ generously to every hole whenever a screw or bolt is removed. You'll never completely keep the water out but you can treat the wood so it's nothing that fungi or bacteria will want to eat when rot conditions become a factor. The diagrams below show a small and large power boat with wood-cored decks.

Core Diagrams
Click on pictures for close up view.
Close Core Diagrams
Deck Cores
Decks are most often cored with plywood or end grain balsa. Occasionally foam or a plastic core system is used, but usually not. The diagram shows some typical cores, and a couple of photographs of a power boat deck showing the wood core.

Once the deck or cabin core decomposition becomes severe you'll know it: Things get a bit springy up there. Mild decomposition may not be so evident, but can be detected by pulling bolts/screws and inspecting the holes, or by tapping the deck and listening for tonal differences. If there a lot of air under there, it sounds hollow. You can also test drill through the deck to take a look. More often than not you'll be right so the test hole is no issue. If the wood turns out to be good, squirt it with some CPES™ and then after the CPES™ cures fill in with an epoxy filler. Our Fill-It™ dries just off white and smooths nicely.

Repairing Rotted Cores

Method 1: Removing Deck Skin and Replacing Core

How you repair rotted cores will depend on a lot of variables: How large the rotted area is, what it is, where it is, where the boat is and what kind of weather protection it has - and how patient you are.

There are two basic problems: 1) Accessibility to the deteriorated area and 2) drying things out. Epoxy will not effectively penetrate wet wood; the wood has to be reasonably dry. So, obviously, the best solution is always to pull the top off the deck and scoop out the bad wood and replace it. Easier said than done, right? On sailboats, especially, this can be a nightmare, with all the hardware scattered around. Still, it might in the end be worth it because things happen a lot faster this way. You treat the edges of the bad wood area with CPES™, and the new wood if it's ply, and then re-install. With balsa you must put it in place first because the backing compound is dissolved by the CPES™, or you can order the balsa core without the film backing. For bonding use our Layup & Laminating Resin™, but wait until the CPES™ on the treated wood has cured. In a perfect world you would replace with a plastic honeycomb instead of wood, but we've been told many times that plastic core material is hard to get in limited quantities. Below is a rough schematic for removing outer glass deck skin. There are various ways you can accomplish this task; this is just one of them.

Removing Outer Skin

Core Removal Diagram
  • Plan and mark your cut wisely. Plan a cut with future restoration in mind. Use straight lines. Consider placement of non skid overlays and deck fittings.

  • Set cut depth to glass thickness only.

  • It may be necessary to work skin free from core with the edge of a putty knife; apply heat to loosen further if needed.

  • Try to remove the skin intact. It can be reapplied over the new core.

Core Removal Diagrams Removing Inner Skin

On some boats, especially the smaller ones, it is sometimes possible to get at the deck core from inside. Removing the inner skin is the preferable way of gaining access to the damaged core because it leaves the exterior surfaces unmarked. The schematic (right) shows steps for inner skin removal. Once again, old remaining wood and new wood core material should be CPES™ treated and bonded with our Layup & Laminating Resin™.

  • Outline the damaged area with straight lines and using a paper pattern transfer this outline to the inner skin. Line up exactly by measuring from permanent deck fittings or bolts.

  • Drill an exploratory hole to determine how far the underside of the outer skin is from the surface of the inner skin. Fit a circular saw with a carbide-tipped plywood bit and set the cutting depth to slightly less to allow for some variation in this dimension. Cut around the outline.

  • Finish the cut through the core with a razor knife. If the top skin bond is completely broken, the cutout will drop out. If not, find a loose corner and pry down, then use a sharpened flexible putty knife as a chisel to free the rest of the core. Heat applied to the outer skin may help.

Core Replacment Diagrams Replacing the Core

  • Chisel the damaged core from the inner skin or use a utility knife (for balsa or foam), or a saw (for plywood). Carefully and thoroughly clear the inner skin of any old core material.

  • Make a paper pattern of the empty section to cut new material.

  • NOTE: it is important to use core material the same as the original, both in type and thickness. The exception to this would be the plastic honeycomb core material--which can be mixed with other types. It should be the same thickness, however.

  • Dry-fit the core into the cavity, trimming as necessary.

  • Wet out the surface of the old core and skin where the new core will bond with CPES™. Allow 1-2 days for CPES™ carrier solvents to evaporate.

  • Wet out all bonding surfaces, both new and old with Layup & Laminating Epoxy Resin™ thickened with talcum powder or colloidal silica to the consistency of mayonnaise, and put new core in place. Brace or weight the core in position, and allow to it to cure.

  • Wet the old and new core with CPES™ and allow 1-2 days to cure.

Replacing the Deck
How you replace the outer deck skin will depend on how large the area is and how it relates to the hull structure. Ordinarily the skin section can be placed back over the new core with L&L Resin bonding and the cut edges later filled with our Epoxy Filler and then sanded smooth. However, if you are concerned that this will not present the same strong structure as it was originally, we recommend you use the steps shown below to restore the deck. Use only epoxy resins for re-bonding, and we can suggest our Layup   Laminating Resin™ as being an excellent choice. It has a long pot life, is a simple 1:1 mix, and remains slightly flexible after curing. After major repairs you may elect to cover the deck with one of the non-skid flexible coverings. It'll hide all your cut and scratch marks.

Skin Replacement (1) Fit outer skin over new core.

(2) Bond with thickened L L Epoxy Resin. The core should be solidly bedded in the thickened epoxy, and epoxy should squeeze out the cut line all around the new section.

Cure thoroughly (at least 24 hours), compressed with weights. (Place polyethylene plastic sheeting between weights and skin).

3. After resin mix has cured, sand/bevel seam areas into a shallow v-shaped depression with the original cut line in the deepest part of the 'v'.

4. Cut fiberglass cloth into narrow strips (or use fiberglass tape) and laminate them into the depression with straight L L Epoxy Resin. Each strip should be about 1 inch wider than the previous one.

Finally, sand the cured surface, then paint with a water-barrier coating or cover with non-skid flexible covering.

Repairing Rotted Cores
Method 2: Treating Core Without Removing Fiberglass Skin

Many of you will elect to repair the core without removing the deck skin. On smaller areas especially this will be the case. Sometimes, though, the glass skin is just too difficult to remove, or, it's an old boat and you just don't want to expend the time and effort. It's possible in these cases to perforate the outer skin with drilled holes, dry the area underneath, and then inject with Clear Penetrating Epoxy Sealer™ (CPES), Layup and Laminating Epoxy Resin™, and then close the holes with Fill-It™ Epoxy Filler.

The key here is drying the core material. Epoxy will not displace water in and around wood cells. Although CPES™ will partially displace light moisture, in almost all instances it will be necessary to use some sort of drying method. It can take awhile. Ideally, this is a project that can be started in the winter when boating is less frequent. The deck can be opened, covered, and then left to dry out over a few month's time. If slow drying is not possible, there are ways to speed up the process. Here's the procedure:

Drilling Diagram
  • Define the deteriorated area. We suggest first tapping it out and then going back with a drill to verify. You want to find the edges up to the good wood and mark the area.

  • Once defined, drill the area with multiple holes* no further than 5" apart. The more you drill the quicker the drying. Be careful not to drill through inner skin.

    *NOTE: We use drilled holes, but one of our web readers (thank you, Jeffrey Manosh) suggests slits cut with a circular saw across the deck, that is, side to side--beam to beam, which would allow more air through, easier application of the CPES™ and L L Resin™ and better ventilation for cure time. The slits need not be all the way across and can be staggered. There would be enough deck left to supply good lateral support (most fore and aft support on boats is provided by the hull), and of course, the slits would be filled in with Fill-It™ Epoxy Filler when treatment of the core is completed.

  • Drying methods: a shop vac can help remove bulk water, flushing the cavity with acetone* will help carry away moisture, blown heat from a hair dryer or heat gun helps, and finally, tenting under a heat lamp can work on a long-term basis.

    *NOTE: ACETONE IS FLAMMABLE. Use caution if using both acetone and heat.
Drilling Diagram

Epoxy Injection
  • When core seems dry enough, flood the holes with CPES™. Our syringe and needles can be useful here. After the application of the CPES™, wait at least a week. You need to allow time for the carrier solvents to evaporate. Due to its highly fluid consistency, the CPES™ is going to reach wood that a thick epoxy resin cannot. It adds significant protection to the core.

  • Next, flood the holes with the Layup and Laminating Epoxy Resin™. A turkey baster works very well here. The L&L Resin is very slow-setting and has time to settle into small spaces. It also will always retain a slight degree of flexibility. It will never shatter. The L&L Resin will cure hard in 24 hours, and will cure at temperatures as low as 28 degrees F.

  • Compress the area with weights immediately after application of the L&L Resin. You can use a sheet of polyethylene plastic under the weights. It will peel away without adhering to the L&L Resin.

  • Close surface holes with Fill-It™ Epoxy Filler. Paint or cover with flexible decking.