I would like to read owner stories of relining and/or coating leaking fuel tanks as to what was involved and what did it cost.
There was a gentleman on the T&T List (some of you may remember that original boating list) who had a great deal of experience maintaining his 1985 CHB Taiwanese trawler and kept detailed records on his web site. Unfortunately, he is no longer with us and his web site no longer exists (this was back in the early 2000's). However, I still have all of posts from that list, going back to 1998 until it closed in 2021 and I've checked his posts to see what he recommended; actually he had several solutions.
No matter what method is used, the basic requirement if doing it from the inside of the tank is to get it as clean as possible. For this, he cut a 14X14 inspection port in the side of the tank (after emptying all the fuel, obviously), then spraying it was a soap and then a degreaser solution multiple times, sucking out the residue as best as possible. Make sure to clean the baffles on both sides, as well.
On one of his tanks he had a welder close all the pinhole leaks. On another he used a sealant which was sprayed on the interior (I haven't been able to come up with the product, but there are certainly a number discoverable on the internet which would be impervious to diesel fuel). Ventilation was a key, and when welding from the inside he kept the exterior cool with a piece of wetted plywood. One of the tanks had to be raised from it's bed in order to perform the work (probably the welded tank).
I actually talked with him about the process at one point in 2004/5 when I was trying to figure out what to do with the fuel tanks on my then 1986 trawler (Hampton 35 - a 25 production made in Taiwan). He favored the spray coating as being easier, and for him it didn't seem like a big deal. I felt otherwise, and opted to remove the tanks and have exact duplicates fabricated from stainless steel.
At the time, 2005, the two 100 gallon tanks cost $1000 for both. The total job (removing two engines, removing the tanks, replacing everything) cost less than $4,000.
Ten years later I did the same type of thing with our 1987 DeFever 41 which had a single FL135; the old tanks had to be cut out of the boat as the boat was obviously built around the tanks. When fabricated, divided each tank in half lengthwise and vertically when they were rebuilt by taking a 1" rectangular section out of the design for the original tanks (only lost 10 gallons per side doing that) and fabricated out of heavy-duty stainless steel which would meet CG specs for passenger vessels. Did a lot of other work in the process including upgrading a lot of fuel lines, CG double-crimped line fittings, new vents, etc. Turned out the original tanks on the DeFever were advertised as holding 400 gallons total, but the new tanks (almost exact replacements minus the 1" section) held a total of 528 gallons (total of both tanks). The cost of the four new tanks, heavy SS, cost a total of $10,000. Labor for just the tank work probably ran about $20K.
A lot of the work is documented on our web site (which is still up) at MVSanderling.net/Blog if you're interested in taking a look there.