• Sealing a Cored Hull or Deck.

    Whether you are installing a thru-hull or mounting a windlass the most critical part of the job is protecting the balsa core encapsulated within the fiberglass layers. Boatbuilders began using balsa wood years ago in the manufacturing process because it is lightweight, adds strength and is low cost. At first the manufacturers only used the balsa coring on the decks and flybridges and as time progressed they moved on to begin coring the sides and finally the bottom. You can search the internet and find an unlimited amount of cored hull horror stories. The purpose of this article is not to discuss the validity of using balsa wood in boatbuilding and I will leave that up to the experts. The message I do want you to understand is that if the balsa coring is not properly protected the inevitable result may be disastrous. Attachment 419

    In the picture above you can see an example of a balsa cored deck. Essentially you have two sections of fiberglass with the balsa wood sandwiched in between. The balsa wood does do a good job of providing strength and stiffness while saving weight. The problem with this layup is when water is allowed to penetrate through the fiberglass the process of destruction begins.

    The most common warning signs you will see on decks is delamination or sponginess. Delamination occurs when the fiberglass separates from the coring layer underneath. 'The fiberglass layer may rise or buckle and if tapped with a mallet will have a popping sound instead of a thud. Sponginess is a sure sign of core problems. The level of decay can only be determined by inspection and will require drilling out a sample. Rotten balsa core has a unique acidic smell that once you have sniffed it you won't forget it.

    There are other various problems that occur with cored decks and hulls but the intent of this article is to help you avoid these problems.

    Dealing With Cored Decks

    Some manufacturers are better than others when it comes to boatbuilding. Don't assume that since you have a new boat that you are protected from core isssues. If you want to find out how well your boat is manufactured just remove some of the screws from the railing and examine the holes. If you don't see signs that a sealant was used in the holes when the railing was installed, you might want to remove all of the screws and reseat them with a good sealant. ANYTHING that penetrates the fiberglass on the boat should be well sealed to prevent moisture from penetrating the coring.

    In most cases a good sealant is sufficient for most installations located on the deck when small holes (less than 1/4 inch) are used. If the installation is for an item that will be subjected to repeated stress or loads it is prefereable to "bed the core" with epoxy resin as described later in this article.

    How do you know what sealant to use?

    Silicones - have more flexiblity and generally less bonding strength, they work well as a seal between objects that squeeze or rely on compression (like a gasket). Silicones retain their flexibility for long periods of time and work well on plastics. They do not work well below the waterline and cannot be painted. 3M Marine Silicone is an example.Attachment 420

    Polysulfide Sealant - synthetic rubber that retains its flexibility yet has a tremendous ability to bond to anything including you and is very messy. Once cured it can be sanded and painted and makes an excellent caulking compound. It can be used on wood, metal, fiberglass but cannot be used on plastics, plexiglass, lexan, PVC or ABS. Polysulfides have a long cure time of a week or more. Life Caulk by Boating Life is an example.
    Attachment 422
    Polyurethane Sealants - should be used when you need a more permanent bond. These sealants are more like an adhesive than a caulk. They are excellent for below the waterline use such as thru-hulls, however like polysulfides do not use them on plastics, plexiglass, lexan, PVC or ABS. Cure times vary depending on the product but average 3-7 days. 3M 5200 is an example.
    Attachment 421

    Dealing With Cored Sides (above waterline)

    The most common items associated with cored sides are the thru-hulls used to discharge liquids overboard such as those used for the bilge pumps or sink discharge. Most of these are located sufficiently above the waterline that a good silicone sealant will be sufficient to seal the thru-hull without the need to "bed the core". If the thru-hull is going to be permanent and made of one of the metals then a polyurethane sealant will be a good choice.

    Dealing With Cored Hulls

    When you are dealing with anything below the waterline in most cases it is preferable to accomplish a procedure known as "bedding the core". This process is also commonly used on other areas of the boat where additional strength is desired. Cleats, railing and lifeline stanchions on sailboats are some of the common items.

    The process is fairly simple, remove the balsa core and replace it with a paste of epoxy resin and a filler. Thats it! It is preferable to use Epoxy Resin and not the Polyurethane Resin found in the local auto parts stores, the epoxy resin is stronger and less likely to absorb water. For a filler colloidal silica is a good choice for the same reasons as above.
    Attachment 423

    Once you have the hole drilled you want to remove the balsa core from the perimeter. The deeper into the hull you remove the balsa core the stronger Attachment 424the area will be. If your intent is to just seal the area around the hole then removing 1/2 inch of the core is sufficient. Folks have been successful in removing the balsa coring with ice pick, others have used a bent nail with the head cut off inserted into a drill. Personally I have a dremel with a carbide cutting bit that has a triangular shaped head that works well. The key is to remove all of the coring down to the layer of fiberglass so you can get a good bond. If the item is going to be secured with bolts ensure the coring is removed from around the hole enough to fully surround the bolt. Next make sure all of the dust and debris have been removed and then use acetone to clean the perimeter of the hole.

    For the next part of the process you will need some rubber gloves, facemask, popsicle sticks, small plastic container, epoxy resin and silica. The popsicle sticks can be used to mix the resin while in the plastic container and assist in applying the paste. The epoxy resin and hardener should be mixed in the ratio suggested by the manufacturer in approximately 1/4 cup quantities at a time. Until you get the hang of it, mixing larger batches will likely result in the resin hardening on you before you get a chance to use all of it. So begin with 1/4 cup mixed thoroughly and slowly add a tablespoon of silica and continue mixing. Once you have the silica thoroughly mixed check the consistency. In areas where the resin could run out of the hole before curing, mix it to the consistency of peanut butter (a paste). This may take several tablespoons to achieve the desired results. In areas where you need it to flow the consistency of syrup will work fine. Silica is similar to talcum powder and the slightest puff of air with send it airborne. Use a mask while dealing with the silica and do not ingest it into your lungs, it is harmful.

    When the paste is the desired consistency you will need to work quickly to prevent it from hardening before you get it into place. Using the popsicle sticks and your rubber glove covered fingers force the paste into the perimeter of the hole. It is important to ensure the paste fully penetrates the cavity you created by removing the core. if you are working with a circular hole start at a point and work in one direction around the perimeter of the hole forcing the paste in as you go. This will help prevent trapping air between the paste and the balsa coring. If the paste is not quite thick enough and begins to slip out of the hole, blue painters tape works well to help hold it in place until it cures.
    Attachment 425
    For epoxy paste depths of 1/2 inch or less you can seal the coring with a single application of paste. For depths greater than 1/2 inch it is preferably to accomplish the process with several layers of paste, allowing each layer to fully cure before adding the next. The epoxy resin generates a large amount of heat while it is curing, if the depth is too great the heat cannot escape quickly enough and the resin will crack. If this happens you will have to remove the offending layer and re-apply. If you have to add additional layers, first sand down the cured layer before adding a new layer on top of it. This will help the new layer bond to the fully cured layer.
    Attachment 426
    When the paste has fully cured it should be sanded smooth to remove any rough edges and to conform to the shape you desire. After sanding clean the area again with acetone and you are ready to proceed with installing your equipment.

    View this video showing an example:


  • Recent Forum Posts


    Grind back the bad area to good substrate and 4...

    Grind back the bad area to good substrate and 4 to 6 inches past. Then rebuild the area with fiberglass cloth and epoxy. Fair smooth with cabosil, prime, and paint.

    ben2go October 18th, 2018, 13:04 Go to last post
    Rob Orr

    Bottom Delamination

    I have a 2000 Caravelle 210 Center Console. I used it for many years in the Great Lakes and brought it to Florida when we moved in 2011. During the winter of 2018 the bottom on the port side...

    Rob Orr October 17th, 2018, 16:27 Go to last post

    Oh. The House bank is 4 Trojan T105 for 450AH. It...

    Oh. The House bank is 4 Trojan T105 for 450AH. It drives a 2.5KW Freedom inverter/charger as well.

    dwsdolce October 9th, 2018, 22:12 Go to last post
  • Twitter