SJ23 Tech Tip G08, (Updated 2011-07-23) Bob Schimmel

Index

Install a Deck Fitting.

INDEX - WEST bonding, Updated bonding, Backing plate, sealant
Generally the first sign of a badly installed deck fitting is a leak.  The leak may be difficult to locate, but can you usually trace it to a rusty through bolt fitting.  The problem arises from the original installation where, if the installer didn't accurately drill the holes, the bolts don't share equal loading.  The overloaded bolt hole elongates, starting a leak.  When you read further on you will discover that bedding the fitting with a marine sealant is not always a solution as the fitting may move under load and may leak again.  The problem is compounded with a cored deck (SJ23 is cored) that can absorb moisture, becomes soft and delaminates.  This is generally the start of a major repair!  Plywood was used to reinforce the deck of the SJ23 where a factory fitting would be mounted.  You will find it under the aft part of the deck as well.  The cabin top is mostly balsa core except under the mast step.  This is why you should epoxy the inside of a fitting hole on an SJ23.

THE WEST BONDING METHOD - A technique developed by WEST involves bonding the hardware with epoxy to distribute the load over the deck.  By using maximum contact surface area, stress is distributed & minimized, and leak-causing movement is gone.  It requires more time to bond a fitting, but WEST claims it performs better without leaking.  In the illustration below are four typical examples of bonding to a deck.  The technique is the same with all of them.  First drill an over size hole.  Then clean out the sides of the hole as shown on the left. Paint the core inside the hole with unthickened epoxy to seal the soft wood.  Thicken the remainder of the batch (peanut butter consistency) and fill the hole.  Insert the screw and let the epoxy cure.  If you wish to remove the screw in the future, coat it with a release agent before inserting it in the epoxy.  This will not reduce the hold on the fitting.  If there is no backing plate to distribute the stress, take this opportunity to add one.  Make sure it is rounded at the corners and tapered at the edges.  Be creative and hide the nut to improve the looks of the cabin ceiling.

NOTE - A suggested alternative to the "Allen wrench" in the drill chuck above it to bend the head a long nail 900.  It can remove debris quicker and smoother with more control.  However, Dremel now makes bits specific to this task, making this job easier. 

This is an excellent technique for installing a fitting on an SJ23 balsa cored deck.  The epoxy will strengthen the deck by bonding the upper and lower layers together and add compression strength for the fastener bolts.  If you also add a backing block and seal the fitting with butyl rubber, all the better. 

UPDATED WEST BONDING METHOD - WEST no longer mentions the previous methods as taking too long.

  1. Overdrill the hole to enlarge it a bit.  A hole can be through wood, solid polyester, balsa cored polyester.

  2. Clean the debris in and around the hole and wet the wall with unthickened epoxy to fill the tiny voids.  Let soak in.

  3. Tape over the bottom of the hole and fill with epoxy, thickened with WEST 404 High Density Filler. 

  4. Alternatively fill the hole with epoxy, thickened with finely cut chopped strand mat fibre to make a putty.
    (To make the chopped fibre, tightly roll up a piece of mat like a cigar.  Then using sharp scissors cut off 1/8" slices.  The short strands do an excellent job of thickening the epoxy into a putty that is amazingly tough.  What you want is a thick but spreadable putty). 

  5. If the thickened epoxy is very thick, fill the hole to flush.

  6. Once hard step drill a hole to install your bolt.

  7. Seal the hole and bolt with a quality sealant.

  8. This technique is stronger than the original polyester resin.

 TOP

BACKING PLATE - A fitting that is fastened with nuts and washers may be ripped out of the deck when it is subjected to a high load.  A proper backing plate will distribute the forces and hold the fitting in place.  Sharp corners on a square block create high stress areas and are to be avoided. The round corners with tapered edges distributes the forces evenly.  If the plate is made of wood, seal the wood with tung oil, varnish or resin to stabilize the wood (prevent warping, cracking, rot, etc).

NOTE - Never apply a sealant to the back side of backing block unless water can drain out of the bolt hole.  Hardwood backing blocks I installed in the 1990s are still in perfect condition.   TOP

BUTYL RUBBER TO  SEAL A FITTING - While Sikkens Sikaflex or 3M 4200 are excellent to seal a fitting to a deck, you should also consider using butyl rubber.  It can be applied to a fitting and tightened immediately because it flows out quite slowly to fill every nook and cranny.  Then it solidifies to uniformly support the fitting.  After 24 hours you can cut or rub off the excess that oozed out.  If Sikaflex or 4200 is applied to a fitting, you have to snug up the fitting loosely then tighten it 24 hours later when the sealant has hardened.  If you squeeze it too thin, it creates insufficient support.  During the cure time you cannot load up the fitting.  Butyl rubber is the same adhesive that holds a car windshield in place.   Consider the following advantages of butyl rubber:

  • Stays pliable, even after 30 years.  It is the sealant that Clark used in the "hull to deck joint" of every San Juan model.  Go ahead give it a nudge.  You'll find that it is still pliable.

  • More flexible than cured Sikkens Sikaflex, 3M 4200 or other polyurethanes.

  • Sticks extremely well to clean gel coat, plastic, aluminum, bronze or stainless steel.

  • If the surface is sanitized it is doubtful that the seal will ever leak.

  • Excellent flow out to uniformly support a fitting for even load distribution.

  • Easy to remove in the future.  Cleans up with a rag dampened in mineral spirits.  (DO NOT use it around a gasoline fitting).

Prepare the hole drilled through the fibreglass by bevelling the top slightly.  This prevents the gel coat from cracking and creates a small void where the butyl rubber can ooze in to create a very tight seal.  To seal the bottom of a screw head, wrap a cone of butyl around the underside of the head and sink the screw in to full depth.  DO NOT twist the screw.  Instead, hold it still with a screw driver and tighten the nut from the bottom.  The rubber will flow to fill the bevel.  Tighten it ONCE ONLY and forget it. 
To seal a fitting clean all mating surfaces with acetone, cover the bottom of the fitting completely with butyl rubber, insert the mounting screws (with a cone of butyl rubber under each head), and set the screws into the deck holes.  Tighten the nuts from the bottom while preventing the screws from turning at the top.  Rub off the excess butyl that flows out from under the fitting.  Wash off any smeared butyl with a rag dampened with mineral spirits.  That's it.  No sticky mess to clean.   TOP

 

__________ DON'T USE THE FOLLOWING OLD TECHNIQUES  ___________

THE ORIGINAL METHOD for F.R.P. (1960) - Drill a slightly over sized hole. Coat the inside of the hole using an epoxy or polyester resin saturated pipe cleaner.  This seals the surface.  Insert a rigid tube (H below) that is liberally coated with resin.  (This tube is a guide for the screw and is there to take the compression load of holding the fitting to the deck).  Let cure.  Use marine sealant on the bolt and base of the fitting to create a water tight seal. 

This technique works, since some of my fittings from the 1990s are still water tight.  However, it is cumbersome and not as strong as the WEST technique shown above. 

THE DRILL & SEAL METHOD - The 'quickie' technique of attaching a fitting is:

  • accurately drill the holes
  • liberally coat the holes, the bottom of the fitting and the underside of the bolt heads with marine sealant.
  • Insert the bolts and tighten the nuts. 

The problem with this technique is that it is structurally weak.  If you over tighten the nuts as shown below, you compress the core as in diagram B, causing damage.  Bad idea! 

It is very easy to over tighten the nuts which results in compressing a balsa cored deck.  I tried this technique once but it leaked in a few years.  Very annoying and quite honestly, a waste of time.   TOP

 
REFERENCE - Installing a complicated deck fitting is such a diverse subject that it is beyond the scope of a tech tip to adequately describe all the details required for your specific installation. 
  1. Above all, the wood core MUST be sealed to prevent the ingress of water.
  2. The core MUST NOT be crushed.
  3. The fitting MUST be bonded or sealed to the deck. 

Regardless of which technique you use, installing a fitting on a cored deck must be done correctly.  Therefore, I suggest you visit the WEST SYSTEM web sit to learn more on the subject.  Or browse their other web site dedicated to Epoxy Products for Building and Repair.  Download their Product Guide or User Manuals.  All excellent stuff.  If you intend to perform this job, order one of their publications for reference.  I own "Wooden Boat Restoration and Repair" and "Fiberglas Boat Repair and Maintenance."  Very excellent publications and where I get a  lot of my ideas from.   
 

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