Originally Posted by Last Tango
The hole in the tank is probably tiny. the foam will seal it ant the integrity of the rest of the tank will prevent the foam becoming saturated.
You're equating this to the problems with the early "unsinkables" but the scenario with the foam enclosed in a more or less sealed tank that will not be subjected to continuous immersion is different.
There are also "closed cell" foams that are commonly used for buoyancy now.
Personally. I wouldn't like and "empty" petrol tank under my deck.
What is the OP tank made of, glass, stainless, plastic, aluminium. That could change the fix considerably.
I'd sleep much better at night knowing it was fixed properly.
It would be interesting to know what foam you used last tango. I am thinking of experimenting with foam on a different project but there are not many supplies available in U.K.
In a can expanding foams are generally not closed cell, the ones that are, only are if their outside membrane is not damaged, they are also moisture cured which means in a tank he might end up with a solid outside and a cake like mixture in the middle. Porable 2 part foams in U.K. Are mostly low density and most are destroyed by solvents and fuel. Vibration causes them to break down and then loose closed cell properties. If the tank is stainless or aluminium any moisture trapped could cause crevice corrosion so may lead to an accelerated decay. Any bodge is gonna de value it, and by the time someone has spent money on a posh pu foam that is closed cell, solvent resistant and then he has hidden all evidence of the filler and breather, it would be cheaper to just learn how to fix it. A hole in a tank is a hole. No matter how big it is, I wish the previous owner of my boat new how just one little hole into the structure of the boat would cause so much damage. A water logged boat is almost a right off eventually.
If the tank in OP boat is grp or plastic and it's a small leak. There are some resins that will patch and hold for about £5. Plastic padding leak fix works, and Scott badder sell ethanol safe vinylester resins. Depends what it's made of, how big the leak is etc etc and how consiencuius the owner is ultimately.