Cure Balsa Core problems with NON-DESTRUCTIVE, Polyurethane Injection

The traditional cure for a bad balsa core is very destructive. You either cut out the laminate above the core or below the core, replace the core and then replace the laminate. This is not only a nasty, costly job, it rarely produces a product that is as good as the original. It is impossible to achieve a fiberglass bond as good as the original and very difficult to make the new work look as good as the original.

A balsa core may last forever if it stays dry but drill a hole anywhere and you have the potential for water to get in, become trapped, start rot and start delamination. Luckily today there is an easy way to detect moisture in a core before damage is done and a way to fix many cores without destructive surgery.

The way to detect moisture is with a moisture meter. A good meter will set you back about $500 but that is cheap compared to paying for the damage of undetected moisture or the cost of buying a beautiful boat that is full of moisture.

The way to fix a core without destructive surgery is through polyurethane injection. Thin, self leveling, polyurethane sealant can be purchased very cheaply and injected under a thousand pounds of pressure with a $7 grease gun. Once the polyurethane cures it is as strong as the original core. This assumes that the fiberglass laminate is structurally sound.

Where is moisture likely to be found in a core?

  • All through hull or through hull/deck fittings. 90% of the through fittings I’ve tested have moisture around the fittings, some as much as an 8 inch radius. A 4 inch radius of moisture is common. That is an 8 inch circle.
  • Teak decks with about 2000 screws are a very common problem. Here you have 2000 potential problems. If you must have a teak deck and your deck has a core, get rid of the screws. Pay whatever it takes to have a glued deck installed. The screws are a time bomb.
  • Any holes drilled in a laminate whether it is for load bearing hardware or a simple snap has the potential to leak. Obviously, the more stress the fastener gets, the more likely the core is to take on moisture and fail.

How can moisture in the core be detected?

  • Use a good moisture meter. Read the directions thoroughly. These meters work very well and they do not lie unless you run into metal under the laminate and that is usually obvious when you look on the underside of the laminate.
  • Check the most likely trouble spots first. That would be the area around all of the through hull/deck fittings.
  • Look for stress cracked areas where the core may have been weakened. Test for moisture there.
  • If you have a teak deck to deal with, you have to remove the head liner to test the laminate from the inside.

When can a core simply be dried out instead of injected?

  • If the core is not rotted, it can be dried out. To determine if the core is rotted, get at the back side of the laminate and drill a grid of 1/8” holes in a 1” checkerboard arrangement. Of course you are not drilling through both sides of the laminate. Drill only through the inside. Do this over the entire area that has a moist core. To get a neat, consistent pattern of holes I apply sticky back cabinet shelf paper. It comes in a checker board design.
  • Clean out the flutes of the drill every few holes. Unrotted balsa will have no smell or smell like resin and it will be fibrous when you roll it in your fingers. Rotted balsa will smell moldy and have more of a mushy or dusty feel. If you are really unsure, drill a ½" hole and pick some of the balsa fibers out with a tweezers. Rotted fibers will crumble or mush. Good balsa will be much harder to remove and it will be very fibrous.

How can a core be dried?

  • If you are lucky enough to live in a northern climate, where the temperature moves above and below freezing in the spring and the fall, nature will dry the laminate after you’ve drilled the hole grid described above. Here, ice crystals form as the temperature drops and the moisture is driven out of the wood. Ever look at a loaf of freezer burnt bread with ice crystals collected in the plastic bag? This occurs when the temperature inside the freezer fluctuates above and below freezing. What you want is a freezer burnt core in your boat.
  • If you are not in a northern climate, or worse you are in a high humidity climate, I cannot help you. Text books talk about injecting alcohol, using tents and halogen lights but I have not used these methods.

When can a core be fixed with polyurethane injection versus traditional reconstruction?

  • If the core is rotted, you need to fix it or the lack of core support will lead to much bigger problems. The question is when can you use simple injection versus tearing out the rotted area and replacing it?
  • If the upper and lower fiberglass layers are structurally sound, the core can be repaired with injection. If the fiberglass does not show stress cracks or obvious hydrolysis (blisters or milky looking fiberglass), it will be fine.
  • Large areas with stress cracks may be repaired with injection but I have not done that so I cannot predict the success. However, I can tell you this. I did do what was to be a temporary repair to a 30 foot Pearson Flyer that had a 6 inch crack in the hull. The outer hull had been punctured from a bad cradle pad. That was 10 years ago. When we injected the polyurethane from inside the hull, the polyurethane oozed out of the hull crack on the outside and then set up. The intent was to do a permanent repair after the racing season ended. A final repair was never done because the area around the temporary repair is stronger than the surrounding original hull.

High pressure polyurethane injection

Some jobs need high pressure injection and a very thin polyurethane sealant. Basically these are places where you cannot remove the core. Here, a grid of 1/8” holes needs to be drilled in the back side of the laminate. Or, you may be filling the hollow space within a rudder.

The good news is that Vulkem 45, a thin polyurethane sealant, is available from Home Depot at a fraction of the cost of tradition thick, marine polyurethane such as 5200. Vulkem 45 comes in quart cartridges and it is thin enough to go through a pin hole. You can fill a handheld grease gun directly from the cartridge of Vulkem 45.

Vulkem 45 is good for injection made under foot such as injection into a hull from the inside. Vulkem 45 can also work for large vertical injection such as a rudder. But for over head jobs use Vulkem 640. It is sold in gallon pails at a professional construction material dealer. Call Vulkem for a dealer location. Where as 45 is self leveling, 640 is the opposite. It will hang overhead yet it is quite thin.

Buy a cheap grease gun from an automotive store. For the grease gun buy a POINT fitting at an automotive parts store such as NAPA and replace the normal grease fitting. Pump caulk into each of the 1/8” holes in your grid until caulk is oozing out of the hole(s) ahead. You may need to plug the hole(s) you’ve already done with golf tees. Buy a few bags of tees.

Use a grease gun filled with Vulkem 640 or 45 and inject it into the core. Each 1/8" hole will take about 1.5 pumps from a handheld grease gun before you meet resistance. If you do not meet resistance, just keep pumping until sealant is coming out of at least one adjacent hole. You will hit some voids in a core that take a couple dozen pumps. These are spots where there was no core originally. This is typical on corners. Or the core may be rotted and you are compressing the rotted core material with the sealant. Once the sandwich has been dried and injected, water cannot reenter because the sandwich is solid. As a bonus, once the sandwich cures, it will feel much more solid. Note, it may take several months to fully cure.

Low pressure polyurethane injection

If you have removed the core, there is no need for high pressure or a thin polyurethane sealant. For example, if you have removed a 1 ft diameter circle of core material around a through fitting, the void will be easy to fill. Use 5200 or Vulkem 921 above the water line. Note, 921 is only available at a commercial construction material distributor.

Removing a core around a through fitting is tedious but can be done. You will be able to remove the core for about 6” around a through fitting. To do this, you can make a routing tool from a flexible screwdriver extension attachment made by Vermont. Use a cutting wheel on a Dremmel to cut teeth in the end of the Vermont screwdriver attachment that normally holds the screwdriver bit. Then put the flexible screwdriver attachment into an electric drill and route out the core sliver by sliver.

For a through hole fitting, after the core is removed, drill three holes at the edge of the removed core in a triangular pattern. Then wrap the through hull/deck fitting in enough cellophane so that it is hard to press it into the hole. Force it into the hole and inject caulk into the first of the three holes until it oozes out of the next hole, plug the hole and move on until all three holes are under pressure.

Note, it takes weeks to months for the polyurethane to fully cure. Polyurethane cures in the presence of moisture so it will eventually cure. A partial cure is good enough to screw things back together. Wait a few weeks before doing that.

After the core is dried can the air holes drilled under the deck be left open?

  • If she is a racing boat without a headliner the air holes can be left unsealed. But if a headliner is installed you must seal the air holes. Here’s why. The wet core was a potential problem because of possible delamination from rot or ice. But now that the core is open to air another problem can occur, mildew. Mildew requires air and moisture. If a headliner is installed and a deck leak traps moisture between the headliner and deck, mildew is sure to occur. Mildew is a sure way to delaminate any structure but you can avoid it if you cut off the air by filling the air holes. While you are doing that you may as well fill any voids in the core at the same time.


Stan Sroga



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