Fuel Tank - Paint/Treat the bottom of the tank? Ideas please

[mattnj]

I removed the tank and noticed the ply floor underneath is saturated in Diesel...
After a steam clean and a gentle shot blast, there is about 10 tiny pin prick holes in the bottom steel plate....is has rusted through from the bottom upwards.

I have just cut the bottom section out, ready to weld in a new piece (i have gone for 4mm plate vs 3mm on the original) not really a bid deal....it is 34 years old.

Question is, does the inside need treating with anything once its been welded (welding will be from the outside) or just fill up with diesel....and what to treat the bottom with, the bit that sits on the floor?
what paint/treatment should I use on the fresh mild steel plate before it sits on a bit of plywood again (which could get wet/damp as it's below the steps) for the next 30 years?

 

[vyv_cox]

If you can add a sump and drain to collect water the tank will last even longer (assuming you drain it occasionally). Internal treatment should be unnecessary. Externally the same type of treatment that you would give a keel - grit blast, coat with zinc rich epoxy.

 

[NormanS]

If it was possible to alter, or do away with the ply base that the tank sits on, so that there was an air space between the tank and the base, it would be better. Other than that, do what Vyv suggests. Sump and plenty paint.

 

[Birdseye]

Sit the tank on a sheet of polythene. As for the inside, dont bother. In 30 years you are unlikely to have the same boat. What I would do though is to put a tap in the bottom so you can drain off any accumulated water / diesel bug easily.