About a year ago we had a volcanic eruption.
Our rainwater catchment tank is 15 years old, was in good condition, and has had no problems until now.
It has begun a relatively small but difficult corrosion problem.
Sulphur gasses are probably the cause of its starting.
I am getting ready to fix all the problems, then repaint it.

The tank is a 10k gallon [~38k L] tank, about 18-1/2'[5.64m] dia X 5'[1.52m] high.
It has a plastic [polyethylene, I believe] liner that appears to still be in good shape with no leaks.
The sides are painted, galvanized, corrugated steel, bolted together with the top layer overlapping the lower layer.
The staining has always felt dry, never wet.
It may only happen at night during very high humidity.
It has been suggested that weeping between the liner and the sides may be the origin.
I'm guessing, but I believe the overlap is no more than 6" or less.

RIMG0274_sm.jpg

I need to seal and stop the corrosion between the sheets before painting.
An automotive anti-rust in spray cans has been suggested.
I'm still worried about corrosion above the sealant growing and breaking the seal with time.
I do not want to drain the tank, then disassemble its walls to treat it, if possible.
If I have to go that far, I may just as well replace the liner, maybe tank.
There must be a way to fix this tank without destroying it.

Thoughts and suggestions on stopping and sealing the corrosion?
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