It’s surprising how much damage a very small amount of water ingress can do to a glass fibre hull or deck never mind a wooden boat! Over the last three months we have had our eyes opened on a number of occasions due to water damage such as a complete new transom required on a sixteen foot power boat, see the remains of the transom in the image below, new mast step reinforcement wooden pad needed under the deck and 'ceiling' laminates on an Albin Express, over sixty chain plate bolts needing to be replaced on a ketch rigged Oyster and then there is our very own ‘IT Andy' who had his issues when his Autohelm went AWOL. The boat hit the quayside and with the force of the impact his forward lower shroud U bolt came out of the deck like one of Kim Jong- Un’s dodgy rockets from its underground silo.
If you discover a water leak, and I know from bitter experience how hard they are to trace (it may well start a couple of metres or more from where the damp patch is showing internally), don't under any circumstances rely on a quick fix, for example a dab of silicon round the offending deck fitting. Yes it is ok as a temporary fix till the end of your hols and if you must till the end of the season but get it repaired properly as soon as you can.
If moisture does get under a laminate be it from a fastening, impact damage etc, wood reinforcement pads can soften and rot, stainless steel bolts can rust through due to crevice crack corrosion and fail with disastrous results i.e. the mast coming down or a rudder falling off. If you do trace a leak back to a fitting, the only 100% safe way of fixing it is to remove the fitting and fastenings and, if the substrate is sound and dry, re-fix after removing any contamination/old sealant from the surfaces. If the fixing hole has not earlier been countersunk (see diagram below) do so before you apply your sealant to the surface of the fitting, the depression area and the thread, messy yes but you can always clean any surplus with acetone on a rag.
Sp Eposeal is excellent as a deep penetrating sealer as is either SP106 or West System 105. Should you find it impossible to dry out completely your damp wood, assuming its not completely rotted, use G/flex epoxy from West System; its also great for repairs and bonding difficult to glue plastics.When bolting fittings onto glassfibre, first drill the correct size hole and then use a countersink; this then gives us a greater surface area for applying sealant. The result… no leaks and no issues five years down the line! For an adhesive/sealant I like either Sabatack XL (good range of colours inc Mahogany and Teak) or Sikaflex 291i available in black or white. If only a sealant is required, Geocel 201 is a high performance flexible polymer sealant, the modern replacement for polysulphide. Available colours are black, white, grey and teak.