The bane of the Taiwan trawler style.
CC RIDER is a 1981 CHB that I purchased in 1985. Since I do all of my own maintenance, she and I are intimately acquainted, probably in the Biblical sense! I digress. Knowing well the proclivities of these boats I have babied the tanks over the years, to little avail.  Noting however that there was no rust on the top of the tanks from deck leaks. This spring the two port side tanks sprang minute leaks in their bottoms.  To remedy this I took the following steps:

1. Pumped all fuel from the tanks.

2. Opened the inspection hatch that is located on the side and wiped out any residual fuel with paper towels after scooping all the crud out with the bottom half of a 2 liter bottle.

3. Liberally splashed the tanks with liquid detergent and washed the tank with water that was then pumped to a barrel on the dock. This requires surprisingly little water. This was done three times to make sure that all scum and fuel residue was removed. You must get to the far side of the baffle!

4. Any oil in the water in the barrel may be skimmed and the water dumped.

5. I then placed an electric fan to blow into the tank to remove any fumes, this was done overnight.

6. The tanks were then cut loose from their mount and raised about four inches, which was the maximum room available. This was done using a crow bar and a number of small blocks of wood cut for the purpose.

7. On one tank I could see the source of the leak and used my 4.5-inch De Walt grinder with a wire brush wheel to get rid of the rust and paint. This was  done to all accessible seams and wherever there was rust.  With the tank exposed I filled it with water and looked for any additional leaks. Since there  were only two and both were readily accessible to the welder he merely welded them.

8. To protect the bottom of the boat from the heat of the welding I merely placed a piece of plywood right under where he was welding. Of course a water hose was kept at the ready and was used to cool the tank as the paint burned and caused smoke. The water kept the smoke down to a manageable level, which was blown outside using several fans.

9. The other tank was a different story. This one required a 14 inch square manhole be cut in the side, the baffle be cut out and then all seams welded from the inside of the tank. Depending on space constraints, possibly two holes may be required.

10. A 3/8 steel flange was welded inside the tank around the manhole. I then fabricated an aluminum cover and drilled and tapped it to match. This was the most tedious work.

11. The tanks were then primed, painted and dropped back into place.

12. End of Portside side story.

13. The starboard side presented a different set of problems. Of all things on the first trip subsequent to the foregoing repairs the starboard tanks
sprang leaks, go figure!

14. The forward tank was not accessible for any type of work on it. These like the others measured 64 inches long and about 24 inches deep by 28 high. They would not come out the available hatch without moving the generator set so it was necessary to cut 24 inches off of the length of each tank. The same procedure was followed as above as to cleaning and cutting.

15. Upon removal they were easy to repair by welding any pinholes and scabbing in steel plate where necessary. A number of places were patched like this.  New ends were welded on, fill pipes moved and the like. Three tanks were made of the two with some loss of capacity. They were sandblasted coated and painted prior to reinstallation. The "3rd " tank was "sistered" to one of the others so as to not require a separate deck fill, though it has its own vent and does not rely on the vent for its mate.

This or these methods are not touted as cure alls but they did work for me. Of course new aluminum tanks could have been built or plastic ones purchased.  I found that only 55- gallon plastic are readily available.
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In closing I was surprised at the amount of scum in the aft tanks as compared to the forward ones. I attribute this to the fact that it is my custom to
run the aft tanks dry first and then the forward ones. This leaves them low on fuel for extended periods for as soon as the forward tanks are low I refuel.  Thus we have more condensation and the result is more water and scum formation. I am scrupulous about always using a biocide and changing filters every 200 hours. I have never had a filter clog following this regimen.