Osmosis in water tank

[lumphammer]

I have just taken off the water tank lid to sort out a leak and as is normal in these situations I now have a bigger problem, that is, lots of osmosis blisters in the gel coat in tank and lid with black mould spots. A new rigid tank is not an option as it wouldn't go through the cockpit locker hatch. I'm not keen on flexible because of the likelihood of chafe so I'm stuck with repairing.

My plan is to grind out the blisters and fill with epoxy filler. sand everything smooth and water proof using 3M Scotchcote 165PW https://www.promain.co.uk/product/3m-scotchkote-165pw-urethane-coating-id9830.

Is there anything else I should be looking at? Do I need to thoroughly dry out the tank (as for osmosis treatment on hull)?

 

[pvb]

Is it really a "bigger problem"? A lot of water tanks have osmosis, but it isn't a structural weakness and with a reasonable throughput of water it shouldn't pose any health problems. A bit of black mould isn't a problem either. I'd just sort the leak, dose the tank with purifier and get on with using it.

 

[PTB]

Are you sure its osmosis? My grp tank had 'blisters' but nothing in them at all. I ground them away, abraded the tank inside, stuck a fan heater on low blowing through the tank for a day of so, and painted it all with Antel water based epoxy. That was 4 years ago and no sign of any recurrence. The antel stuff seems really good and damp tolerant... not sure if the skotch product is ok on grp ?? Although its flexibility sounds a plus point.
If its full blown osmosis, someone will be along to advise. P

 

[Rivers & creeks]

A flexible tank inside your existing tank will have almost zero chafe. I've lost two to leaks and they were both rubbing against fittings, but inside your tank it would sit nicely.

 

[GrahamM376]

One of the grp tanks on our last boat had large blisters and some were leaking. Ground them out, left a fan heater on low for a week or so, filled with gel filler on the inside and abraded and glassed patches on the outside.