The AC's fuel tank is constructed in the old traditional way, from steel coated in 80/20 solder, and then soldered together. Presumably, that means that a fire will cause a fuel leak! That is all the more reason to keep your precious AC safe from fire risks. The good news is that the solder coating preserves the steel tank very effectively, so that reduces one potential risk of leakage.
If you have not dismantled the car as part of a restoration, then there are a few bits and pieces to remove before you can get your hands on the tank. The plywood partition in the boot (trunk) needs to be removed. Mine had previously been cut into two parts, probably by AC themselves, making it much easier to take out. Remove the rear seat and the trim from behind the backrest. I'll leave it to your initiative as to how best to drain out the fuel, noting that earlier ACs had the fuel filter mounted lower than the tank, on the bulkhead, making it a useful drainage point.
Remove the rear shelf trim to reveal the two coach-bolts that the tank hangs from. Remove the 4 nuts from the front of the tank, and disconnect the rubber hose to the filler. It helps if you can unstick the two hose-clips from the filler-hose, after which the filler-cap complete with hose can be lifted out. Remove the outlet pipe (90 deg. bend). Taking out the gauge sender-unit, also gives a little extra room. The tank will pull out through the boot, although this might turn into a lengthy wrestling match! The boot-lid will need to be either removed, or opened out horizontally.
The outside of my tank was originally coated with bitumen, which I scraped off with a heavy duty scraper. Don't use any power-tools to remove the paint/coating, because of the lead in the solder coating.
Old petrol left to stand, deposits a varnish. This varnish crumbles to a fine dust when dried out, which resembles rust, so don't panic thinking the tank is rusting away, when a pile of reddish-brown powder comes out!
I'm not dancing with R2D2's little brother. I'm cleaning out the petrol tank.
To clean the inside, I used some POR15 Cleaner Degreaser, diluted, plus some old nuts and bolts to rattle around in the tank to dislodge rust and varnish. I sealed all the openings with heavy-duty tape and turned the tank around, shook it, and left it to stand on each of its six sides in turn (at least 20 minutes each side). After draining out the cleaner, I kept repeating until clean. I have a borascope, and was able to examine the condition and cleanliness of the inside.
This isn't a sunken ship wreckage. It's the inside of my fuel tank after cleaning, with a baffle visible to the left. The horizontal solder seam (on the front side of the tank) can be seen to the right.
The tank outlet pipe incorporates a double gauze filter. This consists of an extremely fine brass gauze wrapped into a tube, with a coarse gauze outside it, and a brass cap to seal off the top. This was largely blocked up, so I dipped it in that same POR15 Cleaner Degreaser, and within a few minutes, it came out shining. The very fine inner gauze was already slightly damaged, with larger holes through it. I'm not sure if it is worth repairing, since it is so prone to blockage?
I gave the inside of my tank the same phosphoric acid treatment as I do for steelwork that I paint. I stuck with the POR15 brand and used their "Metal Prep". The procedure was similar to cleaning out the tank, with all internal sides getting a treatment for at least half an hour each. Sloshing the liquid around, helps to treat the two baffles in the tank. Extra care needs to be taken, since this is an acid, so wear goggles and suitable gloves. I found that a small pile of rust flakes came out afterwards.
I decided not to use any kind of tank sealant. It is not needed unless the tank condition is fairly poor - perhaps with some pin-hole leaks. Applying tank sealant presents a challenge, since you need a very clean surface inside, but obviously have a limited view and access. A borascope would be really handy for this. If the sealant doesn't stick well, then it can cause serious problems if it comes loose.
Fuel gauge sender unit
The float can be cleaned up with the same cleaner used above. Hopefully, the float pivot will unseize with some penetrating oil and very gentle persuasion. New sender units and floats (plastic - ugh!) are available at the time of writing (2017).
Painting the outside
The 80/20 solder coating doesn't present any problems for painting. You can treat it with that same phosphoric acid solution prior to applying your preferred painting system. In my case, it was epoxy mastic followed by 2-part polyurethane. The tank should be given the best protection to guard against external damage or deterioration. I didn't top-coat the filler pipe, because this will be flock sprayed (as original) to match the boot.
I was already aware of a tiny leakage where the filler pipe is soldered to the tank - fortunately near the top of that pipe. This could be clearly seen while cleaning the inside of the tank, since some cleaner seeped out of a pin-hole in the solder join. Rather than try and use epoxy tank repair material, I thought I might as well rely on the epoxy-mastic that was going onto the tank anyway. It is thin enough to work into any such tiny hole. I'll monitor it for any leakage in service.
Finally, the hessian padding can be stuck on with paint. This padding cushions the tank against the aluminium frame that helps to support it.
Fitting the tank back into the car looks straight-forward, but can take some time. With the hessian padding, it is a tight fit in the aluminium support frame. The nuts at the front of the tank, allow you to adjust the tank's position until the top mountings line up. There may be some plywood packing pieces to refit, before the 2 coach-bolts go back in.
When I refitted the outlet pipe, I could not screw in the 90 degree bend, because the damper subframe obstructed it as I turned it. Since the end of the pipe had not been cut square, I filed it square, which gave the needed clearance. The damper subframe had been a later addition to this early AC. The coupler hose for the filler, is a standard length, 75mm. Inside diameter is 60mm, with a wall thickness of 4.5mm. It is reinforced and it would seem prudent to obtain one that can resist modern fuel, although it might be hard to find hose of such large diameter? The fibre washer for the filler cap can also be renewed.
A problem for classic cars that are not used frequently, is that varnish forms in the fuel tank and system, and this leads to blockages. Condensation in the tank might also lead to corrosion. There are additives available to reduce this problem, although I can't give any first-hand experience of how effective they are.