Cut a suitable inspection hatch/opening in the top of the tank. Take the cut out piece to a metal fabrication shop and have a cover plate fabricated of similar aluminum or stainless steel
so that it overlaps the cut out hole with significant surface for a secure seal against the existing tank. If the tanks is not of sufficient thickness to afford a rigid surface after cutting the inspection hole you will have to have the fabricator make a ring of stainless steel
to fit inside of the tank. This might be the easier way to go because you could also have the inner ring fitted with a number of studs or bolts, drill corresponding holes in the tank top, add at least two holes drilled and tapped to allow securing the inner ring to the inside of the tank to hold it in place when the inspection hatch is not bolted on.The two fasteners for these should be with chamfered machine screws with a chamfer cut in the existing tank top to allow them to remain flush or some other means of holding the inner ring in place when the hatch isn't in place.
This will allow access to clean and inspect the tank. Should you determine that the bottom, or any other portion of the tank leaking or appers that it may be, there is a product sold by Casswell Plating, Lyons, New York
that can be used to seal the tank. This product is designed to seal leaking gas tanks. I have used it several times to salvage
tanks that are difficult to replace, mainly on vintage motorcycles but also have used on a boat tank. The instruction for the use of the product direct a through cleaning
, rinsing with solvent and then sealing all openings and pouring the mixed two part epoxy
like material into the tank and rotating the tank in all directions to coat all of the interior
with the honey consistency goop. You can call Casswell or email
them and get their opinion on the use of this material using a paint
brush to spread it inside your tank. I am fairly sure this will hold up to diesel fuel
and will give you peace of mind that your tank will not leak - ever.
I have also used this stuff off label to coat a leaky oil
pan - inside and out. Perfect solution as far as I was concerned.
Casswell plating website https://www.caswellplating.com
Good luck and safe sailing