It starts with a small leak around a piece of deck hardware or even a pinhole in a resin-starved layup. Water seeps between the layers of fiberglass, rotting balsa and plywood cores and turning them into mush. Even rot-proof foam cores can be progressively delaminated by sailing stress and the repeated expansion of ice during freeze-thaw cycles. There are few things that send a potential buyer scampering or start an owner sweating faster than a soft deck.
The traditional cure is to peel the deck open from either above or below, replace the rotten core, and relaminate and finish. This is expensive to contract out and demanding of skills for the DIY. A lot of sailing time can be lost. What if we could simply inject something into the space to replace the collapsed core and rebond the skins? We’ve read about epoxy injection in books, but it runs counter to every bit of conventional wisdom about surface prep. No way around it, you’re sealing a wet, ruined core in place.
But looked at a different way, do we really need the ultimate bond? The core is a relatively weak material; foam or balsa vs. glass or carbon fiber. Perhaps an expanding foam that bonds to the skins is strong enough. As for dampness, epoxy requires less than 12 percent moisture in wood, resorcinol less than 15 percent, but Gorilla Glue and polyurethane expansion foams can tolerate up to 30 percent moisture, which is what you are likely to find deteriorated core. Polyurethanes like Gorilla Glue use water as their curing agent. While we’re not suggesting injection to repair highly-stressed areas or high value boats, there are many aging boats with soft decks that could use a little stiffening without breaking the bank or triggering a major rebuild.
We also need a glue that is compatible with wet conditions. Realistically, you are not going to get the damaged core very dry. So epoxy may not cure or bond properly.
How We Tested
We bonded fiberglass skins to both new and uniformly rotten plywood to create test coupons. The rotten plywood was dry when bonded, because the core would have been dry when first created. After they had cured, we soaked them for a day and then allowed them to dry for two days before moving on to injections. We also laid-up a virgin balsa core coupon for comparison. We then injected the rotten samples with a variety of adhesives. Both the controls and the test samples were then tested for stiffness and strength.
We repaired a stand up paddleboard we found on the rocks. The skins had been punctured multiple places, the underlying foam crushed, and everything was soaked and growing algae. We looked for project boats for testing, but the damage was never uniform enough for comparison testing, so we relied on the synthetic test panels described above for product comparisons.
Finally, we repaired a ruined foredeck hatch with a rotten balsa core. We’ve been jumping on that for a year.
What We Tested
Two adhesives in our lineup should be familiar to most sailors. West Systems 105/206 is a popular combination from Gudgeon Brother, used for a variety of boat repair projects. The original Gorilla Glue is omnipresent in hardware stores. Inject-a-Deck is described as a structural marine foam and adhesive that claims to cure as hard as wood. According to the company’s website, the product was developed by a sailor who was involved in the aerospace industry.
|Product||West Systems 105/205||Inject-A-Deck||Gorilla Glue Original|
|Strength (Pounds)||80 lbs.||50 lbs.||123 lbs.|
|Comments||Shear failure at top of glue pillar||Shear failure at thickest void later||Shear failure at top of glue pillar|
|Number of injection points||10 injection points||3 injection points||5 injection points|
|Size (Ounces)||152 oz.||48 oz.||8 oz.|
Strength. All of the products would pass a walk-on and light stomp test, because the deck is supported in two dimensions, a far stronger structure than the narrow beams we tested. However, all of the injected coupons were far weaker than virgin plywood laminate, which was several times weaker than the balsa cored beam.
Composite decks generally fail when the core shears. Although plywood seems like a strong material, it does not work particularly well in a composite structure. Because wood fibers are not as stiff as glass fibers they can’t effectively share the tensile load created during bending. For comparison, imagine a nylon rope tied alongside a chain; no matter how strong the nylon rope is, the chain does all the work until it fails. Thus, the only important stress on the core is shear, created when the two skins try to move in opposite directions as the panel bends, and plywood is weak in shear. Foams range from very weak to moderate, and balsa is one of the best materials available for shear loads.
Stiffness. The injected coupons were 10 to 20 times stiffer after injection, but not as stiff as the balsa coupon. Under bending stress, the core is also subject to compression loading, as the skins try to move closer to each other. Even before the coupons failed, either the columns of glue would flex from side-to-side, voids would compress, or the relatively soft injected foam would compress. However, you’ll probably only notice the difference if you compare it to a section of good deck, or if it is in a highly loaded area, such as under a stanchion.
The kit came complete with everything we needed, other than common shop supplies. The injection gun is smooth and strong, and the motionless mixer tips very effective. Within a few minutes after injection the foam expands 6-8 times, depending on the space available and the moisture content. It cures within an hour, and if you go to revise the repair later, it will be easy to remove, no more difficult than removing other core materials. Only three injection points were needed for a 10-inch sample (4-inch spacing). Because of the strong expansion, this was more than enough for this heavily rotted plywood, and the 8-12 inch spacing recommended by the manufacture seems about right for most deck projects.
The down side is that the foam is weak and friable. The Inject-a-Deck coupons were ten times stronger than the floppy coupons we started with, and we’re pretty sure that a repaired deck well supported by beams could withstand vigorous foot traffic for a while. But they were weaker than samples repaired with Gorilla Glue or epoxy, five times weaker than the virgin plywood coupon, and 14 times weaker than the balsa coupon.
We’re also concerned that the expansion of the foam can force the skins apart, since it has nowhere to go. Two of the samples were placed under weights, to prevent the pressure from forcing the skins apart. We thought this might simulate the way good sections will hold bad sections together. In fact, the foam lifted the weights off the ground, exerting an expansion force of 2-4 psi (576 pounds per square foot). Some of the foam will come back out the hole you injected it through, but if the deck is weak, expect some bulging. The instructions say to walk on it to force it back down. (This didn’t seem very practical to us, given the force exerted.)
Based on our testing, this high-expansion foam will fill huge voids, minimizing the required number of holes. But it’s messy to work with, provides minimal strength, and the pressure caused by foam expansion may further delaminate the deck and even make it bulge upwards and downwards. It will stiffen the deck, but it will not restore the original strength and toughness.
Bottom line: We would only use this if the boat was otherwise headed to the dumpster.
Our repair process varied by product and was based on the manufacturer’s guidance.
For epoxy, we used 10 injection points on each coupon, in two rows on 1.5-inch centers. Because the epoxy does not penetrate far, more would help, but that’s a lot of holes.
For Gorilla Glue, we used a single row of five injection points because we thought the moisture-induced expansion would cause the Gorilla Glue to spread farther. Although not as strong as epoxy, the Gorilla Glue foam is very high density and is much stronger than injection foams
Following the Inject-a-Deck guidelines, only three injection points were needed for a 10-inch sample (4-inch spacing). Because of the strong expansion, this was more than enough.
West Systems Epoxy 105/205
Like most epoxy adhesives, the West System formula does not expand and does not spread far from the injection site unless there are huge voids. In effect, you are building a grid of epoxy posts, gluing the top to the bottom. We used the West Systems 105/205 fast hardener system. Our impression was that the epoxy had stopped soaking in before the epoxy kicked, but a slow hardener might give slightly better results. That said, you will spend an hour adding a drop here and a drop there, trying to keep the holes full as the epoxy soaks its way in. We tried forcing it in with a syringe, but it couldn’t be rushed.
We used 10 injection points on each coupon, in two rows with 1.5 inch centers. Because the epoxy does not penetrate far, more would help, but that’s a lot of holes.
Will you be fixing it right later? Foam is easy to dig out with scrapers and a wire brush, often as far as a few inches under good skin. This can be important if the repair is near stanchions or some other complex area. Gorilla Glue will be more challenging to remove but feasible with some vigor. Epoxy will require complete skin removal and serious grinding. We’d avoid injecting epoxy if we felt sure we would be fixing it right later.
Long accepted as a quasi repair method for small problem areas, we reached this same conclusion. The epoxy does not really spread, so the strength of the repair depends on close spacing of holes and the pillars they create. Bonding is poor and would have been worse if the test samples had been wet. Not hard to do, but weak.
Bottom Line: Recommended for very small areas of damage, no more than a few inches in size.
Gorilla Glue (Original)
Based on polyurethane and using water as the curing agent, the wood workers reading this know how Gorilla Glue behaves around moisture. Under overly dry conditions it doesn’t want to cure, much like some polyurethanes. The solution is to lightly mist the surfaces to be joined, give the water 10-20 minutes to soak in, and then apply the glue and join. If there is too much moisture, the glue foams; not as much as a true expansion foam, but enough to force a good bit out of the joint and weaken the bond. Thus, the proper way to use Gorilla Glue is with just enough moisture and enough clamping force to prevent it from being forced out. A foam-filled joint is a weak joint.
Dry the core the best you can. Protect the area from the weather, use fans, heat guns, and even vacuum to pull the water out. Dry is best, but the reality is that core will remain slightly damp. If you really want it dry, you need either a lot of time or to peel it open. Thus, the best glues are those that tolerate moisture. In northern climates repeated expansion of trapped moisture, such as ice, will further damage the core and continued delamination.
Mark the injection points in a grid and apply masking tape. This can minimize any chipping of the gel coat. Additionally, the adhesive will bubble back out of the hole, and drilling through tape makes clean-up and finishing a lot easier. With Inject-A-Deck we recommend draping the entire work area as though prepping for surgery; massive amounts of foam can come back out.
Drill a grid of holes. If the delamination is slight and you are using a low-expansion product (epoxy or Gorilla Glue), drill 3/16-inch holes on 1- to 2-inch centers. The adhesive will not soak far. If the deck is really floppy and you’re using Inject-A-Deck, drill ¼-inch holes on up to 10-inch centers. The foam will travel farther and the ¼-inch holes provide more room for the excess to come back out.
On vertical surfaces, angle the drill downwards so that the glue won’t just run out. Use a clamp-on drill-stop on the drill; you don’t want to drill into the cabin, although if you do, just slap some duct tape on the underside. You need to drill all the way through the core to insure maximum penetration, but you don’t want to puncture the inner skin.
Inject-A-Deck will expand and come out of the holes. Topping off is only required if the voids are huge. With Gorilla Glue, and even more so with epoxy, expect to spend the next 20 minutes topping-off the holes.
Working from underneath is possible with Gorilla Glue. Just slap some tape on each hole as soon as you fill it; expansion will keep it full. This is not as effective as working from the top, but you don’t have to bother with refinishing to cover the holes. We tried it once and it seemed to work.
If you are dead set on using epoxy, we suggest using a slow hardener and warming the deck (not the epoxy—it will exotherm) to 115F; noticeably warm to the touch but not hot. This will thin the epoxy and allow better penetration.
Use a razor to shave off excesses while the adhesive is still green. This will save a lot of sanding. Remove the tape.
Depending on surface damage and the characteristics of the product, you may need to sand, fair, and finish, as we did on the standup paddleboard. Even if the surface is smooth, it will look like a pegboard. Another alternative is simply to drill out the top 1/16- to 1/8-inch of each hole and fill with matching Marine Tex or Gelcoat. Not perfect, but it may work for just a few holes.
It is easy. If the core was plywood or balsa, there is still some rot in progress and you’ll be back in a few years. If the core was foam and the damaged area is small, maybe you’re done.
We used a single row of five injection points because we thought the moisture-trigger expansion would cause the Gorilla Glue to spread farther. Although not as strong as epoxy, the Gorilla Glue foam is very high density and is much stronger than injection foams. However, it spread only 0.5- to 1-inch in each direction, so additional holes should increase the strength proportionately. We used a closer spacing when repairing the paddleboard.
What about long-term exposure to water? Although Gorilla Glue and Inject-a-Deck are rated as waterproof in short term tests, there have been reports of failures of polyurethane foams and Gorilla Glue in outdoor uses. One problem is ice; if water migrates into the pores it will force the skins apart with every freeze/thaw cycle. We also suspect the users may not have followed the instructions regarding clamping pressure and tight joints. We’ve used Gorilla glue successfully on outdoor projects. Will it hold up to long term saturation?
Is the repaired core actually going to be exposed to super high moisture for long periods? We sure hope not. Injection does not arrest the underlying cause of the problem, which is probably moisture intrusion, leading to rot in balsa and plywood cores and freeze-thaw damage of foam cores. Unless you fix the leak, the problem will recur within a few years, or perhaps by spring if it freezes. If you repaired the leaks, the core may see some dampness but should never be soaked.
Our paddleboard, repaired by injection with Gorilla Glue and faired with epoxy, has been knocked round, thrown on the beach and jumped on. So far, so good. It has not been immersed in water for long periods.
Renowned for its ability to bond damp wood (although not always strongly), it had been suggested to us by several readers. It does not expand as much as Inject-a-Deck, but enough to force its way a short distance into nearby cracks and voids. We wonder if plugging or capping the injection points after filling would have improved penetration. It bonded as well or better than epoxy to damp surfaces, and produced three times the strength per injection point. Although not nearly as strong as a good balsa panel, a Gorilla Glue repair could approach the strength of a poor plywood panel.
Bottom line: Recommended for repair of damaged core. After 6-months of immersion testing, it may graduate to our Best Choice status.
If you’ve ever seen the messy, wet mulch removed during a typical re-core job, you’re probably not surprised that our injection test strips were pretty weak. You’re gluing a damp, dirty surface using mulch-thickened resin. You will fill some voids and add some stiffness, but the rot will continue.
If the deck is very spongy, the boat is old, and the alternative is the dumpster, a high density foam like Inject-A-Deck can make the boat serviceable for inshore use in light conditions for years.
If the core damage is more limited, perhaps the result of impact, Gorilla Glue is strong, has enough expansion to fill small gaps and can tolerate damp conditions. We recommend spacing similar to that used with epoxy (a 1-inch grid), which should result in considerable strength.
Epoxy injection didn’t impress us. Holes have to be drilled on very close centers and the result will be difficult to repair properly later. If we were sure it was only localized delamination or a crushed core, and that the core was sound, we might give it a try.
If the damage is in a high stress area, the boat is high-value, you like to doing things right, or you have a sneaking suspicion you will want to do it right later, bite the bullet and do it right the first time. Core replacement is time consuming but really not all that difficult to learn. The finishing can be a bit tricky; maybe you can combine it with repainting the deck.
We’re leaving samples soaking in water for six months to see what happens long-term. We’ll report back.