MENU Member Login
On the waterfront in Downtown Toronto since 1894 From novice to old salt, there is a place for everyone at National Yacht Club.
BoatBITZ #2 – Repairing a Wet Core
August 31st, 2011 @ 08:18 PM EST by admin

By Sean McKee

In our fist BoatBITZ column we discussed how to mount hardware using through-deck fittings. In that article, I touched briefly on making sure that the core of your deck where you planned to mount said hardware was free of moisture. Let’s talk about just such an event this month.

If you will recall, a critical step in mounting hardware is determining whether or not the core of your deck is wet. Some readers will ask: “Core of my deck?” Yes. Most Glass Reinforced Plastic (GRP) boats (i.e. fiberglass) have a core of some kind, usually balsa wood, sandwiched between outer and inner layers of fiberglass; the basic idea being that wherever there will be a significant weight load like the deck the wood gives it more strength.

How it’s made

To better understand how to fix a wet core, let’s quickly review how fiberglass boats are constructed. First, a “female” mold of the deck is sprayed with gelcoat, a 2-part thick paint that hardens like plastic and forms that shiny finish on the outside of the boat. After the gelcoat has cured, layers of fiberglass cloth are applied in opposing directions (for strength) after being soaked in a 2-part epoxy to form the outer structural skin of the boat; without this the gelcoat would crack and flake away.

Next the core is applied; for the purposes of this discussion, we’ll focus on boats with balsa wood cores since those are most prevalent at NYC. The core itself is composed of hundreds of 1-2” square blocks, usually at a thickness between 3/8” to ½”, depending on the application. The squares are arranged to form a sheet and a mesh backer fabric is applied to hold the squares together while the squares themselves allow the sheet to bend in a reasonable approximation of the curvature of your deck. Finally, the inner layers of fiberglass and epoxy are applied, encapsulating the balsa core in a hard plastic “sandwich”.

The Dangers of Wet Core

Most boat manufacturers will apply the balsa without any form of treatment (i.e. “buttering” each individual square of balsa with epoxy), before completing the sandwich construction. This brings us to the reason for “end grain” balsa wood. End grain is used since it will allow any water that may penetrate to travel along the length of the wood fibers and go no further; since the length of those fibers is only the thickness of the core sheet, and then theoretically the water should be contained within the square. Unfortunately, over a protracted length of time, this moisture can penetrate the adjacent blocks and eventually you can be left with quite a sizeable patch of wet core.

Since the core is contained within the fiberglass sandwich of your deck, you may think that there would be no real issue, however depending on the severity of water penetration in your deck, you face the possibility of a significant reduction in the strength of your deck to sustain downward (footsteps), upward (u-bolt connections for blocks), or lateral (winches and cleats) forces on your deck. In fact, there are many cases of wet cores at the centre of significant gear failures since the advent of “plastic fantastic” boats.

Checking for Wet Core

As I mentioned in last month’s column, the best way to determine moisture in your core is with a non-penetrating moisture meter. These are handy gadgets, but at $400 a pop, they aren’t exactly standard equipment in the average boater’s collection of rusty crescent wrenches, broken needle nose pliers and screwdrivers with missing bits. The best way for you to test your deck is through percussive sounding. Use a plastic-faced mallet (available at Canadian Tire for about $6) and rapidly tap areas of your deck. You should hear a sharp “TACK, TACK, TACK!” If you hear a dull sounding “DUM, DUM, DUM” that’s a sure sign of deck delamination due to a wet core.

Planning Your Attack

Now that you have determined that your core is wet, it’s time to bite the bullet and get on with fixing it. Depending on the severity of the moisture and location, you may choose to wait until the off season to make your repairs – take it from me, get these completed as early as possible after haul-out while the temperature is still warm enough for epoxy. Trying this prior to launch in the spring puts you on a compressed timeline and you’ll make mistakes, cut corners or delay your launch date which nobody wants after 6 months of winter.

Determine if you can make the repairs from inside the boat by attacking it from the inner layer of the sandwich. If you have a headliner that is easily removable, you’re lucky and should take this approach to the repair. Some of you will have a molded fiberglass liner which will make things more difficult. Unless there is no other way to repair the core, avoid cutting through the top side of your deck at all costs unless you really know what you’re doing and have the skills to make the repair undetectable from view from the top!

The First Cut is the Deepest

When you’ve located the area of core you want to replace, draw a line around the area of fiberglass you need to cut out – make sure it’s as straight as possible from an aesthetic perspective. If you need to cut out the deck from the top, this step is particularly important. I use a rotary wheel cutter (Dremel Tool) with a cutting wheel designed for plastics for this purpose; it’s fast and makes a very narrow cut. Once you’ve cut all the way around, you should be able to pry off the fiberglass layer to expose the wet core – save this piece if you can get it out without breaking it as repairs will be easier.

Dig out the core. Depending on how wet it is, the balsa could fall out like mud or need a bit of persuasion. A wide and dull chisel works well for this. Once the core is completely out, allow the section to air dry for a couple of days to a week and then sand the area with a “mouse” sander or by hand; you just want to remove all traces of the old balsa and score the surface to give the new epoxy and balsa something to adhere. Once the area is sanded, wipe it with Acetone to remove all of the dust.

Buttered Up

Measure and cut your new balsa core to fit the repair area and do a test fit; finding out it doesn’t fit when the balsa is slathered with epoxy is not what you want to do! Once you’ve confirmed a good fit, mix up a batch of epoxy – for this I use the West System epoxy with filleting powder to thicken the epoxy to a peanut butter consistency. Using a disposable brush or palette knife, “butter” each square of balsa individually – because it’s on the mesh backer, it is easy to just “crack” it and butter the entire length. Keep doing this until all 6 sides of each square are well buttered – when it dries each square of balsa will be encased in hard plastic, forever eliminating the worry that water penetrations will spread beyond one square.

Apply some of the peanut buttery epoxy onto the surface of the cavity as well. Set the balsa into the cavity and press hard – think of setting tile in your bathroom. The repair should stick and hold in place, however if you have done your repairs from inside the boat, be prepared to brace the balsa from underneath to keep it positioned and firmly pressed against the remaining fiberglass. I have used a piece of scrap plywood braced with a 2×4 which works well – just make sure you cover the repair in wax paper before doing that so your plywood won’t become a permanent fixture on the boat when the epoxy dries.

Once the epoxy has dried and you have removed any bracing, check to see if there are any cavities around between the new balsa and existing core. These should be filled with more epoxy thickened to the consistency of putty.

Cover Up

The final step is to add the last layers of fiberglass to complete the “sandwich”. First, make sure you sand the repair area with 120 grit sandpaper to give the epoxy on the glass cloth some tooth to adhere to; wipe the dist off with acetone and you’re ready to glass. For these types of repairs, I use a thicker glass cloth with a combination of cross-weave on one side and chopped roving on the other; this provides the thickness and support you need for mounting hardware or heavy traffic areas.

Cut the cloth with a pair of sharp shears so there is at least 1”-1-1/2” of material that will overlap the edges of the repaired area. Depending on the thickness of your glass cloth, cut 2-4 pieces to size and set aside ready to go. Mix up your epoxy (unthickened) and apply it to the repair area with a disposable paint brush (not a foam brush – epoxy eats them!). Take your first piece of cloth and position it on the repair and use the paint brush to wet out the cloth – the cloth should start to become transparent when it absorbs the epoxy. Repeat this process until all your glass is done. Be sure to eliminate all air bubbles in the cloth which will reduce the strength of the repair.

If you have had to do your repair from the top of your deck, make sure you tape off the section around the repair area with masking tape and wax paper to eliminate the potential of getting epoxy on your deck (this makes you look like a schmuck). If you were lucky enough to have saved the piece of decking you removed, make sure it is sanded with 120 grit sandpaper on the back side and wiped with acetone. Wet out the repair area and the back of the deck piece you were able to salvage; trust me – salvage this in one piece at ALL costs unless you’re a glass and gelcoat guru (of course if you were one, you wouldn’t be reading this article). Apply the glass panel to the repair and weight it down with something (plywood covered in wax paper weighted down with an anchor will spread the pressure evenly across the repair. Be sure to wipe off any epoxy that oozes out of the gap with a rag and acetone!

Once the panel has dried onto the repair, remove all of the masking around the area. Now re-mask an area about 1/8” on either side of the cut line and apply a layer of gelcoat to fill the gap. Re-apply as necessary until the gap has been filled and then sand the gelcoat smooth with wet/dry sandpaper starting from 400 to 120 Grit until smooth and shiny. Remove the masking tape and you’re done (finally).