I have the same problem on my '86 Ericson 38-200. The leaking diesel is causing black goo in the bilge. I am going to empty the tank (about 40 gallons) and give the fuel away, then cut an inspection/access hole in the top of the foremost (largest/deepest) part of the tank and then try to clean & scour the bottom weld(s) of the tank. Then I will pour/coat the welds with epoxy, (or fiberglass resin?). Before buying/installing an expensive inspection plate cover I will test the seal with a couple gallons of diesel. If the tank still leaks, I will empty it then cut the top off most of the tank, remove the baffles, and then install a 20 gallon bladder-tank. I will post results here. Wish me luck!