Alright people, time to get serious.
As most of you know, steel fuel tanks are prone to all manners of oxidation, corrosion, etc. A compound chemical sealant is the way to go to ensure your tank lasts a long time. You'll need to bolt the fuel tap holes shut with suitable bolts and washers or cut some corks to size and really screw them in there. You then need a proper stopper in lieu of the fuel cap. Don't be tempted to use the regular cap as this will be badly damaged by the chemicals. Use rubber or a large cork, or cut some aluminium/nylon/thick cardboard and then duct-tape it over the filler neck. The first part of the process is a rust remover, you basically throw a bunch of old screws and nuts (anything small and pointy) into the tank, add the rust remover and shake it real hard for a long time. You could also use an old chain, as it will be much easier to remove once you're done. This is extremely noisy, annoying and tiring labour but you have to do it right. This is another occasion where it's all about that one-two-three:
Even with a fairly new tank, you'll be amazed at the amount (and the shade) of sludge that comes out once you're done. You then neutralise the chemical with another mystery chemical diluted in water and dry it thoroughly with a suitable heat source. An oxy-acetylene torch is not a suitable heat source. If you have a fiberglass tank you can skip this part and you'll be glad you can, believe me. In theory you're supposed to prop the tank up in 4 or 5 different positions and leave the rust remover to fizz away for two hours each time and then leave it overnight before you neutralise it with the other chemical. I just didn't have that sort of time or logistics so I took a chance and cut those times a lot shorter. The tank has never been used and there was no visible corrosion, so I thought I'd risk it and yes, there was a lot of s*it in there.
Presenting: a corrosive powder!
"Hmm... so, which ones do I mix again?"
Once the tank is properly clean and dry you can move onto stage two.
This is where you mix the remaining chemicals (mad-scientist laugh highly recommended) to obtain a thick resin with which to coat the inside of the tank.
Now comes the really tricky bit: you have to distribute it evenly over the entire inner surface of the tank and I mean every nook and cranny including the "ceiling", or else you'll be back to square one if you leave bare metal exposed anywhere. To do this, you have to constantly, slowly and methodically rotate the tank, whilst keeping an eye on the goop inside through the main fuel port.
Ok, and now... take two!
No, it's not strawberry jam. Or lava.
When you do the ceiling, you will have to put a stopper in and check every few minutes that you got every bit. This dance lasts quite a while before the resin hardens - though this can vary greatly depending on tempterature and humidity, my advice being don't do it if it's -5° outside. A dentist's mirror, or similar implement will help you check those places that are hard to reach. You don't want the coat to be too thick, or it'll never cure properly. That said, it mustn't be too thin either, or it could flake and peel off. You'll know when you've got it right though, it's all a matter of patience and method. The final stage is letting the resin cure, the longer the better. What you end up with is a glass-smooth surface that creates a "tank within a tank". You'll need to clean the fuel tap holes carefully and fit new Dowty washers on the taps.
Thanks to Witold, who was kind enough to host, and provide rugged and reliable transportation:
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