Stop Corrosion

About a year ago we had a volcanic eruption.
Our rainwater catchment tank is 15 years old 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 about 18-1/2' dia X 5' high.
The sides are painted galvanized corrugated steel, bolted together with the top layer overlapping the lower layer.
I'm guessing, but I believe the overlap is no more than 6" or less.

Attachment 381041
I need to stop and seal the corrosion between the sheets before painting.
An automotive anti-rust spray 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 replce the tank.
it don't needs replacing.

Thoughts and suggestions on stopping and sealing the corrosion?

Post was moved to General Engineering
Grahame

I am not sure what you can do. Corrugated iron tanks here went out of favour once concrete tanks became commonplace, and now plastic tanks are replacing the concrete.

However what was done to lengthen the life of the corrugated iron tanks once they started leaking was they were plastered with a concrete mortar which can last a surprisingly long time, I have also seen them fibreglassed which leaked like a sieve afterwards, then took up. Modern corrugated iron tanks are lined with a plastic liner, but I still think they have a poor life expectancy.