CAUTION the following repair is how I did it and it seems to have worked well. If you don’t trust the idea of putting glue inside a fuel tank, then try it for yourself, as I did, on a rusty tin with different fuels in it, for at least two weeks. Make sure the glue has gone completely hard before adding fuel.  Allowing soft glue into the fuel system may cause damage to components.

This is the method I used to repair my S3 tank, which had a very rusty bash-plate underneath and appeared to be rusty inside as well.

1. Use an old filler-hose, two lengths of bar and some vice-grips so that the filler spout can be sealed.
2. Seal any extra holes on the top of the tank - I had made a new hole in mine for a fuel return pipe.
3. Put about 10 litres of water in the tank and add one tub full of caustic soda (about 500g I think) available from any supermarket, or hardware store - very cheap.
4. Clamp the filler-hose and allow the caustic to work for about an hour and then roll the tank on its side and leave for another hour. Repeat until all sides are done.
5. Drain the solution, following safety directions on the caustic soda container.
6. Hit the inside of the tank with a fierce jet of water. I used a length of 1/2" copper pipe clamped to the garden hose with a bend in it and the end flattened, to increase the pressure.

Heaps of foul-smelling, brown gunk came out, leaving a nice silver tank! It also freed the jammed level-sensor arm and unblocked the end of the pick-up pipe. This is now the time to unsolder the rusty base protector if it has one. To do mine I placed it on top of an old drum on its side. The holes in the base of the bash-plate and on the ends are where the manufacturer applied soft-solder; a gas flame will melt the solder and allow the plate to be prised loose with a screwdriver. Make sure that you wear boots and long trousers, as molten solder is rather painful as well as being poisonous. If petrol has been present and not washed out by a specialist, then do not attempt to unsolder the base; I got away with it safely because the petrol had converted to a nasty brown sludge with no flammable aromatics and the caustic soda had got rid of all that. Do not take a chance with your life! On my tank most of the soldered spots were already broken by the rust, so it did not take long to get the plate off – even so, I had removed every orifice-cover, so that if something did want to burn, the gases could escape without an explosion; nothing nasty happened.

The old bash-plate.

PVC glue as used on PVC drain-pipes, makes a very good sealer; I have tested this on a rusty tin with petrol and diesel and it works superbly. Thanks to Ian (Back In on Aulro.com) for this really useful tip!

1. When the tank is clean, remove the sender unit and pick-up pipe and seal the holes with some sheet-metal plates and silastic. If you don’t remove components such as these the glue will jam or block them  and stop them working.
2. Add about 1 litre of glue to the tank and seal the inlet hose with the vice-grips.

Surprisingly the glue only leaked out at one point on the front face where the bash-plate stops, so the tank was in much better condition than it looked. Initially I thinned the glue with acetone but the glue then seemed a bit runny and didn’t cover everywhere, which indicated that I hadn’t used enough the first time and that I had drained the excess too soon. So I repeated the process with the glue un-thinned and left the tank on its sides for longer. It is best to do this on a perfectly level surface so that the glue spreads well. I inspected the inside with one of these inspection cameras. http://www.machineryhouse.com.au/Search?S=inspection+camera
Be careful handling the glue and read the safety directions first; getting it in your eyes will lead to permanent blindness!

Total cost for everything so far about $20 - the commercial kits cost over $100!

A new bash-plate was needed, so I had to buy some new galvanised steel. Murphy was around though, so I made it too short and had to weld in the piece I left out! The radius on the bottom-front of the tank is about 25mm so I bent the sheet in my cobber’s press using a bit of round bar, and a press block made from timber. Similar press blocks can be made from steel flat bar welded to a base and suitably buttressed, to bend plate from about 3 to 6mm.

Experimenting with a piece of scrap on the press block; the block is screwed together.

Rather than use solder to hold the bash-plate on, I used Sikaflex Adhesive, which has the advantage of never letting go; easier to do; and hopefully not letting water in as well. The plate was clamped lightly using sash-clamps and pieces of timber, to squeeze the excess glue out.

New Bash-plate glued in place.