Combing Repair

Time: Several Days on-and-off.
Level: Advanced
Bravery Level: 5 Star - This Project has a very high scare factor but failure to do something about rot will ultimately lead to utter distruction of your sailboat...so...forge on me matie, forge on!!!
 
The one thing I missed when I purchased the Christopher-Jin was a loose mooring cleat on the aft port quarter.  I knew it was loose but I didn't know that the plywood between the layers was rotten.  It wasn't until I lifted the seat hatch and reached under that I discovered that the inner layer of fiber glass had pealed back and the plywood core was dissintegrating.
 
This would be my first repair job on the Christopher-Jin.  See the next two drawings below to see the anatomy of the fiberglass skin and what can go wrong.  You can see why it's very important to inspect all bedding and make sure you re-bed any areas where hardware is attached an may have come loose or shows breaks or cracks in the bedding.  The amount of rot was actually much larger than shown in the drawing.  It covered a area roughly 4"x18".
 
I cleared out the hold and crawled in armed with screwdrivers, wrenches and a putty knife.  My good friend Mark worked top-side.  We got the cleat off and the then the two vents.  Then Mark went off to work in the cabin, under the step to inspect the battery compartment.
 
I, lying on my back, removed everything down to the upper/outer fiberglass skin of the aft combing.  It did not take long.  The inner skin and plywood core just fell away.
 
Next I took a piece of craft paper and rough cut it to fit into the damaged section.  I then traced around the edges of the damaged section, creating a paper template.  I had Mark trace around the vent holes and the cleat holes from the top-side.
 
I then cut out the paper template and traced it onto a 1/4 inch piece of ply.  I cut the ply on my scroll saw and then checked the fit.  A little extra trimming and I then cut the vent holes with a hole saw.  I decided to wait on the cleat holes as I decided it would be better to drill those after mounting the new plywood core.
 
Once I got the plywood to fit correctly I used the "core" to trace around a couple of pieces of fiberglass cloth.  I set these aside for later. (Note:  Do not cut these out yet.)  Then I mixed a small batch of slow
curing West System Epoxy, www.westsystem.com and coated the entire core.  I let that cure and then gave it a second coat.
 
Again I checked the fit of the new core.  Then with clamps, screws and block of wood at the ready, I mixed a batch of fast curing West System Epoxy.  I slathered the top-side of the core and the bottom-side of the combing and then pressed the core in place.  Mark set the clamps through the vent holes and then using the wood block, we screwed down through the cleat holes and through the core to squeeze that section of the
                                                                                                      core up against the under-side.
 
We let that cure for several hours.
 
Next came the hard part. I had to cut and Epoxy the fiberglass fabric to the underside.
To do this I over-cut the fabric and then checked the fitting.  Remember to cut out the vent holes.  I wanted the fabric to overlap onto the undamaged section of the combing.  Once the fit was good I mixed a batch of fast curing West System Epoxy and slathered it on the fabric.  Next I brushed a coat on the plywood core and then quickly pressed the fabric onto the core.  The trick here was to keep the fabric up and on the core until the Epoxy fired-off.  I let the first layer cure then I applied a second layer.
 
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