The tank I bought will be prepared thoroughly.
The inside will be completely cleaned and sealed to protect it permanently against rust.
First I drained and flushed the tank.
After that I cleaned the outside and removed the old sending unit.
To remove any residue that has built up on the inside I filled the tank with half a Litre of fresh petrol and added a handful of clean screws and nuts.
Then I closed all the holes and gave the tank a good shake for about 10 Minutes.
You won't believe how filthy the mixture was when emptied out of the tank. :eek:
After that I flushed the tank again to clean it from any dirt and grease left inside.
After the tank had dried out I fought the rust inside ...
I filled it with a handful of clean screws, gave it a very good shake and emptied the screws and the rusty residues.
That step I repeated several times and the picture below shows what came out of the tank.
Here a cement mixer would have come in very handy as discussed in this thread:
http://www.renault4.co.uk/forum/showthread.php?t=5950
After a few times of filling in screws, shaking them around and emptying them out again, the inside became clean of rust and less and less of the rusty powder came out of the tank together with the screws.
Here you can see the dry, clean, and mostly rust free inside of the tank:
(It was a bit of a challenge to get a good picture through the small opening though.)
After that I gave the outside of the fuel tank a coating with cheap rust primer.
It is just a quick shot out of a spray can and will be removed again after the inside of the tank is properly sealed.
The tank was zinc plated in the manufacturing process and therefore needs a special etch primer specifically for zinc plated surfaces.
It's just nicer to work on a clean and freshly painted tank.