March 2003 Telltale Page 7-8 - by Tom Winlow and Marcel Laroche.
Replacing wet cockpit seat core is the easiest form of core repair and well within the ability of most people. There is little risk of making things worst.
Most seat hatches become wet because water gets into the wood core around loose hinge bolts, or because the top glass skin and bottom skin are not properly joined together at the aft edge between the two hinges. Once the core gets wet, it looses some of its rigidity and the seat flexes underfoot. Eventually, the top glass laminate cracks, sometimes separates from the core and the problem becomes worst by allowing water to flow in more readily.
To determine the extent and amount of moisture in the core, use a moisture meter. Failing this, drill 1/8inch holes, spaced one or two inches apart and radiating from the bolt holes. Drill the holes through the bottom skin and core only. If the wood core is very wet, the core material will cling to the grooves of the drill bit and feel wet to the touch. As you progress further away from the source of water ingress, the core material will not cling as well to the drill bit, will be lighter in colour and not so obviously wet. Eventually, when dry core is reached, the core material will fall easily from the groves of the drill bit, will be lighter in colour and will not feel damp when squeezed between the fingers.
Dry the drill bit groves thoroughly between each test hole. If the core is balsa, the wetness is likely to be confined to the bolt hole area and decrease rapidly as you move away from the hole because water does not migrate easily across the grain of the balsa.
If the core is plywood, the wetness will be fairly uniform over the whole area of the cockpit seat, because water migrates readily along the grain and across the grain.
If the seat hatch has been wet for a long time, the core could be detached from the skin. Verify the extent of the wetness and/or delamination by sounding the top skin with a small plastic hammer. A "tick" sound indicates delamination of the top skin from the wood core. A "thud" sound indicates a soft wet core. A "tock" sound, means that the skin is still attached to a still solid core, but it could be damp.
If soundings and drilling indicate that more than half the core is wet or delaminated, the entire core should be replaced. Drilling holes in the core in an attempt to dry it does not work. I have tried it and after 18 months, there was only a small decrease in moisture meter readings. The core was still wet and rotted in some places when the bottom skin was removed.
Clamp the hatch upside down on a flat surface and start removing the inside glass laminate. Using a rotary cutting tool, begin by cutting a small area of the inside laminate around the bolt holes. Using pliers, pull the skin away from the core. Take a small piece of the core and squeeze it with the pliers. You will be surprised at the amount of water that comes out of the core. If the core is balsa, keep removing the inside skin until dry core is found. Remove the core with a dull wood chisel, being careful not to damage the outside laminate. If most of the balsa core has to be removed it is probably better to remove it all and replace it with same thickness plywood, if the seating surface is flat. Replace the core with end grain balsa if the seating surface is curved.
If the core is plywood, the moisture will not be as localized as with balsa, but spread fairly evenly throughout the plywood. It is best to remove it all.
Once the core material has been removed, sand the inside surface of the outside laminate to remove all traces of old core. Soak the inside surface with acetone, let it sit for a minute and wipe clean with a rag. Repeat a couple of times. Acetone mixes perfectly well with water and will help in extracting some moisture from the glass fibers.
If the core was end grain balsa and only half was removed, end grain balsa should be used as the new core. If all the core was removed, plywood can be used if the seat is a flat surface. Good quality non-marine plywood is quite adequate, as this time it will be sealed properly.
Shape the new core material so that the bolt holes are through solid glass only. If water penetrated into the core because the core between the hinges was exposed to the elements (à la C&C 27), shape the core material so that it ends about one inch from the rear of the hatch. This will allow you to tie the inside skin to the outside skin between the hinges.
Before glassing the new core, ensure that it fits closely to the inside surface, prepare your clamps or weights, ensure that your working surface conforms to the seat hatch shape (flat or curved), because when the whole thing sets, the shape will be cast in concrete, so to speak. Apply two layers of masking tape over the bolt holes on the outside skin and cover the work surface with a plastic sheet.
Use epoxy instead of polyester resin, because epoxy is a much stronger glue and has a much longer working time. Mix the resin and hardener thoroughly before adding the thickener (colodial silica). Mix to a consistency of peanut butter and apply to the inside surface with a serrated putty knife as if you were installing floor tiles. Press the core onto this surface and wiggle it to help spread the epoxy evenly. Weigh it down or clamp it down just enough to ensure that the core has an even contact with the inside skin. Use more thickened epoxy to make a 45degree fairing filet around the core. Let the whole contraption set for about 12 hours.
Once youre satisfied that the epoxy has hardened, sand the fairing and the edge of the plywood to remove all sharp protrusions. Sand the gelcoat off for two inches around the repair area, to the point where the glass fibers are visible. Wipe everything clean with acetone. You are now ready to apply the glass cloth. If you are using epoxy, do not use glass mat, because the binder used to glue the random small fibers into a mat material, reacts negatively with epoxy. Also, glass mat is difficult to apply around sharp curves. Glass cloth is much better, as it is stronger, easier to wet and shape.
Cut the first layer of glass cloth about 1/2 inch larger than the repair area and "dry fit" over the area. Once it is properly aligned, pour some epoxy over the middle and gently spread it out in all directions from the middle outward, using a small paint roller. Use a one inch paint brush in areas where the roller wont reach. Ensure that the first layer is wetted thoroughly, by using more resin than required. This extra resin will be absorbed by the next layer of cloth. Cut the next layer 1/2 inch larger than the first one. This will allow the edge of the second layer to cover the edge of the first layer. This will taper the edge of the repair area and will reduce the amount of final sanding. With the help of a second pair of hands, carefully place the second layer of cloth over the first. Without adding extra resin, press the second layer onto the first with the roller, starting in the middle and working outward as with the first layer. Once you are satisfied that the second layer has absorbed all the extra resin from the first layer, roll on some extra resin and apply the third layer. The procedure is the same for all other layers. Build up the bolt hole area to about the same thickness as the core, using bits and pieces of glass cloth. For cosmetic reasons, colour the epoxy resin of the last layer of cloth. When dry, sand smooth and apply one coat of coloured resin for a smoother finish. Drill the bolt holes slightly oversize to facilitate the installation of the bolts. You now have a seat completely sealed from the elements, provided that the top skin is sound, because the bolts are no longer through the core, but through solid glass.
Repairs to cracked topskin laminate, gelcoat cracks, gelcoat colour match, non-skid pattern.