My moisture meter led me to suspect just one area on the boat may have delamination, although the usual test, tapping on the hull sides to detect a hollow sound wasn't very persuasive. To check, I drilled a hole and water came out, so I drilled some more all around the area that read 30 (full scale) on my moisture meter:
The hole at the top left of the image is a drain pipe from my forward hatch guttering. You can just make out a pencil line around the drilled holes which shows the extent of the delaminated area. It seems the pipe has leaked water into the core and it has delaminated an area of less than 2 square feet.
After draining the water out (maybe 50-100 ml), I left the hull to dry. A week later, the moisture meter still read 30. I paid a visit to a neighbouring catamaran where all the gel coat has been peeled off and the hulls left to dry out since last August - with the wetter areas warmed more recently by heat lamps. His wettest areas also read 30 on the meter, although we couldn't get any water out. Still, this suggested that hoping water trapped between the fibreglass and the core would evaporate out of the little holes was too optimistic.
I inserted some toilet paper wicks:
They helped a lot, and at first, the readings came down a little, and then stopped.
I googled a lot, and found that just removing the water won't be enough. The water contains solvents - either salt, or solvents from parts of the resin that has dissolved. If the solvents aren't removed, the water will never go from the core. Imagine trying to dry out a sweater that has been wet with sea water - it will never be properly dry until it has been rinsed in fresh water. And even if you dried the sweater but left the salt in it, it would absorb moisture from the air.
As it happened, I'd just bought a new vacuum cleaner, a Vax carpet cleaner, and noticed that this model has a little water pump in it, so that it is equipped to pump water through a small pipe. Just the job! I added food colouring to the water so that I could see the water behind the fibreglass. The result was spectacular!
The holes I'd drilled just happened to be the same size as the water pipe. Inserting the water pipe in one hole, and sucking with the vacuum on another, the water was plain to see flooding the delaminated area. What was also plain was the the moisture meter had allowed me to accurately outline the delaminated area - water didn't penetrate beyond my pencil line. Using different holes for the pipes, I was easily able to flush everywhere in the area. I then just sucked as much water out as I could get, and put some new toilet paper wicks in. Out of curiosity, I tested the area again with my meter:
I guess the meter reads capacitance of the material it is measuring, which must be related to the conductivity. Pure water is a poor conductor, but water containing solvents is much better. I guess removing the solvents was the cause of the lower readings. Anyway, with now just fresh water behind the fibreglass, I left the wicks in place, with the intention of warming the area later with heat lamps to remove the last of the water. I monitored the meter readings every day, and found to my surprise that the readings went down, then up again. I figured the remaining water had dissolved some more solvent and rinsed again. Again, the readings dropped, and then went up a little. In the end, I rinsed the area three or four times, and now the readings are just slowly falling. After a few days with heat lamps, I'm confident that the core will be dry enough to allow me to inject some epoxy resin, and job done.
My progress was monitored by other people in the yard, and they liked what they saw, so we treated the other catamaran in the same way:
With two of us on the job, I was able to take a photograph of the operation this time! The numbers on the hull are the moisture meter readings. It's not so plain to see that water behind the layup in this photo, but we could see it pretty well. This boat also needed several rinses before the readings stayed down. This cat now has heat lamps on the area, and the readings are coming down - as they never did in the last 8-9 months. Clearly, rinsing is an essential part of treating delamination!
Once the area was dry, I tried sucking epoxy into the affected area. I closed all but the highest and lowest holes off with tape, connected a vacuum to the top hole and a tube full of epoxy to the bottom hole and... it didn't work. The epoxy needs a little filler in it to reduce its brittleness, and it was too thick for the vacuum to pull.
Much better was to use a syringe. I added some red chalk powder to the epoxy, so that I could easily see where it went. That worked great! The are now looks like it is bruised, with the red showing through, but I know it is now well sealed from water, and the skin and foam are properly bonded. It'll paint over just fine!