Rotten deck core was one of the two "big surprises" with which which my boat came equipped. In hind sight, this should not have been a surprise. However, I didn't spend much time worrying about it. The price was right, and I knew I was taking a calculated risk purchasing this boat. It turned out that both of the risks I wagered against bit me on the back side, but in the end it's not so bad. The deck repair, while very time consuming and somewhat unpleasant was not all that expensive.
When I first bought the boat, the decks were filthy. She had been sitting, mostly unused, in her slip for the better part of the summer. I don't mind cleaning at all, so this didn't phase me. The angle of the boat in the water caused much of the dirt to pool in the are of the rearmost stanchions. With about an eighth of an inch of dirt, almost any crack can be made invisible. So it was with the cracks in the deck at the two rear stanchions. After a few aggressive deck cleaning sessions I finally uncovered enough of the grime to see what I was dealing with.
My suspicion is that leaks from the chain plates allowed water to enter the core and over time soften it up. The decks on a CS27 are very heavily built, probably due to the deck-stepped mast. So, there was never a time when I noticed a softness to them. However, the compromised rigidity allowed the upper deck to flex enough at the stanchions that a crack formed.
I was hoping that the cracked area was solid laminate, and my strategy was to grind out the crack, then fill it with new laminate. This little fantasy would have been fantastic had it come true. Unfortunately, I ended up uncovering a nasty, soupy mess of rotten core that left me with no doubt about the need to do a full clean out. Resolved to keep moving forward, I began the long march to removing my toe rails. That's roughly 80 screws per side, all through bolted, many stuck. If you don't have an impact wrench, don't attempt it.
Although it wasn't the right tool for the job, I used my angle grinder at 90 degrees to the deck as a sort of circular saw. It worked quite well, and in short order I had the upper laminate removed to expose the core. If I were to do this again I would probably select a small cordless circular saw with a laminate blade. A large saw would be too much.
The next step was cleaning out the rotten core. The vast majority of it came out with very little effort using a sharp 1.5" putty knife. The pockets of not-so-rotten material which held their grip came out with a scraper end of a pry bar being lightly tapped by a mallet. With all the large chunks removed, I used my 4" angle grinder with a 40 grit disk to clean up the bottom fiberglass laminate. Although messy, this was easy and relatively quick.
The final step was to grind a bevel on the edges to allow a good adhesion for the new top laminate. Given the position of the jib track depression, and the edge of the deck I really didn't have a lot of freedom to design a bevel. It is what it is. I ended up with about 1.5 inches all around. Hopefully that will work.
The new core is simply end-grain balsa. My local West Marine didn't stock this kind of building material, so I had to go with an Internet chandler. I selected Jamestown Distributors because they seem to have just about everything related to hard-core boat building. A few days later (these guys ship fast!) I had the mat. I used some poster board to make templates of the cut out areas on each side by cutting approximately 1 foot sections, trimming with scissors, and taping the sections together.
With the templates in hand, I used a jig-saw to cut the balsa into the rough templates. Use a sharp blade. Back at the boat, I used a Dremel tool with two different bits to perform the final fitting and trimming. A #144 high speed cutter was used to clean out any remaining debris from the edges of the decking, and a #407 sanding drum was used for shaping the balsa.
I installed the new core in approximately 12" sections. It would have been more efficient to keep it long and install all at once, but I recognize and respect the boundaries of my skill and really wanted to avoid having epoxy kick before I was ready. One foot seems to be a nice safe amount that corresponded nicely to the amount of epoxy I'd been mixing.
I used West 105 resin with West Colloidal Silica mixed somewhere between ketchup and Vaseline in thickness. I wanted it thin enough to be able to squirt out excess, but thick enough that it would be hard as a rock and stay tucked into the areas I placed it in. This viscocity also served well at filling in the slightly uneven lower laminate for a good bond. In all the project required about 1 32oz bottle and a bit of a second one. If I had been a little more efficient I think I could have done it with just one.
As you can see in the picture to the left there is a short section, approximately 6" long where the balsa ends and the hole remains unfilled. This is the area where the rear stanchion bolts through the deck. My plan is to fill this area with a laminate rather than balsa to ensure the deck is not compressible in this area as well as to ensure the balsa is well sealed from the bolt holes. I don't want to deal with this again!
There were a few places where the balsa was skewed, or didn't lie perfectly flat despite the pressure I installed it with. For a short time I was kicking myself about it, and debated pulling it out and re-trimming. That's when I had a quick dose of reality. These decks were solid as a rock when they had pudding-consistency balsa in them. This boat was built to last with very heavy lay ups. Having installed brand new epoxy-saturated core will be 100x stronger than it was even if it is imperfect. Don't sweat it. Just do a good job and keep putting one foot in front of the other.
The balsa core went in very easily. I kept a Dremel tool on hand with a drum sander attachment that let me form on-the-fly. All balsa was bedded firmly in epoxy thickened (mayonnaise/ketchup consistency) with colloidal silica, which is just about as structurally rigid as it gets for creating a joint between the existing fiberglass and the new core. I didn't really need to weight it down because the epoxy created a bit of a vacuum in the pressed core. I couldn't get it out if I wanted to, and it always dried very flat. In retrospect, it probably would have been the right thing to do. I would suggest using waxed paper to cover the repair area, then placing trash bags filled with sand/dirt on top, or possibly a plate that was weighted by cinder blocks.
Once in place, I did a pretty straight-forward fiberglass lamination. Because the wood surface was marginally imperfect, I wanted to use a thickened epoxy for gap filling. I again went with colloidal silica for its strength despite the sanding hell it would create. I placed progressively larger width sheets of 10 oz. glass cloth into the repair area until I was within about 1/16" of the height of the gelcoat which remained on either side of the repair area. It got a bit messy on the edges where the cloth curled, but it only takes a kiss with the grinder to clean it up. It took about 6 layers of 10 oz. cloth to fill in the gap. Two layers is pretty darn rigid, so I have absolutely no concerns about the strength of this repair.
I spent a little time dressing the edges of the patches, and then cleaned everything up with a little acetone.
The final step was fairing the repair. I used epoxy thickened with 407 low-density filler. It's annoying to mix, but it goes on very nicely and isn't too brutal to sand. I had intended to use 3M Premium Marine Filler, but after buying two packages of I discovered a warning indicating that it was not to be used over epoxy coatings. What a inconvenient restriction! That's ok, I was able to put it to use elsewhere. The fairing took longer than I'd expected, but with patience it came out pretty nice.
The final step, I'm sorry to say, is outsourcing. I'm had a professional apply gel coat to the area I repaired, and do a best-effort match on the non-skid. My original plan was to paint the decks, but I decided against it for two reasons. The show-stopper was the cost. It's prohibitively expensive to have the decks painted with AwlGrip. On a practical note, I also think I will be modifying some of the deck hardware, and it's easier to patch gelcoat than it is to patch paint. So, Gelcoat it is with a much friendlier price tag.
With the deck finished, I was able to re-install the toe rails, but not before I had put two coats of aluminum primer on them, and four coats of flat black spray paint. Two season later the rails still look new, so don't laugh at using Krylon rather than Interlux.
The deck to hull joint had been cleaned out with solvent and scraping (lots of scraping!) and then re-bedded with 3m 4200. The toe rails were also bedded in 4200, although all other deck fittings are bedded in butyl tape.
I did a minor tweak on the chain plates by filling them out with thickened epoxy to reduce the gap the butyl needs to fill, and then re-bedding them in a pretty typical manner.
All in all this was a very successful project and one that not only preserves the boats value, but improves her sea-worthiness, and my confidence in her.