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The tank is not aluminum, it is gelcoated fibreglass and it is molded right into the boat liner. The top is also gelcoated fibreglass. I want to bring the tank top back to original rather than put in a plastic one which would not be placed in as good a position as the original thus affecting speed/performance. The actual tank is in mint condition, it is only the top that is damaged. I suspect it may have been left full after fall haulout and froze thus expanding and cracking the top. That is the only explanation I can imagine that would damage the top like that. Later on I may install a second tank made of plastic right at the aft end of the quarterberth and only fill it when we are heading out for a week or more thus giving us twice as much potable water.Sorry, don't have the answer to that one. If it were my boat. I'd replace the tank, 'cuz the average life of an aluminum water tank is about 20 years and I'm betting this one is close to that old if not older. The thing about aluminum tanks...the first leak is always only the FIRST leak. It's highly likely you could invest a whole bunch of time effort and $$ into repairing it now, only have it fail somewhere else within a year or two. A top quality rotomolded PE tankfrom Ronco Plastics Ronco Plastics Marine Catalog isn't that expensive and will outlast the boat. Just be sure to specify a water tank when you order it.
The top is already off, that is the top in the photos. I will take a couple of photos today of the actual tank which should make things more clear. It is not a problem to repair the top following the steps in my original post. My only concern is in using the right product to form the seal between the tank and the top when I put it back on. It fastens down with screws about every six inches. The surface of the tank where the top sits on is nice and smooth but the matching surface of the top is by no means a "machine fit" so I think whatever I use to form the seal will have to be something in a tube rather than a roll of gasket type material. 3M 4200 is the only thing that comes to mind but I am hoping someone else who has had their tank top off will chime in here and let me know what they used and if it was successful.Maybe a dumb idea, maybe not: What if you cut the top completely off and made a new "shoe box lid" cover for it, sealed to the tank with epoxy?