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To anyone who has a water tank built in under the sole (floor), I suggest you check the ply.

I found the ply sandwiched between 2 layers of fibreglass was wet (as one would expect in a water tank). When the inspection/filling hole was cut, whoever did the job did not seal the cut edge. Fortunately I had to scrape back only about 6mm of ply. The tank has been empty for about 3 to 4 months and I have tried various methods of drying the ply. Some 2 months ago the moisture content was over 22% (friend's meter maxes out at 22%).
It now looks and feels dry. Next weekend I will seal the edge. Was going to epoxy where i had to scrape out soggy ply, but decided just to seal it, because this way I can monitor it and if needs be repeat the scrape and seal process, if needed.

Just to clarify, the inside top of the tank is FG, there is 12mm ply above this and above the ply there is FG forming the floor of the boat.

I caught it in time and the floor is still good.

John

What Mark do you have John?

Just saw your other post, a Mk 3

I had the same problem, but the underside of the floor had all the fibreglass flake off and leave bare timber. At this stage, I just put in a 100 Litre bladder. The plan is to one day remove the floor and re-seal the original tank and put it back to original. However this one day may be a long way off, as the bladder is doing a perfect job of delivering fresh clean water to the tap.