The filler cap was firmly rusted in position. I had to
force it off and in doing so broke the locking
mechanism. After detaching the plastic lock cover I
resorted to an old screwdriver and hammer to unscrew the
filler cap. I managed to put the cap back together
again so it looks OK, but the lock will never be operational
Removing the cap revealed a very rusty interior to the tank.
I drained the tank into a petrol contained and removed the
petrol tap. This is some sort of alloy or maybe zinc
and would almost certainly be damaged by electrolysis or
vinegar. I found the petrol tap blocked by rust in the
reserve position so I disassembled it. I wish I
hadn't. The internal spring made it a pig to put together
again. I'd recommend trying poking a piece of wire
through before taking it to bits.
I wrapped some plumbers tape around an old pencil and used
this to seal the petrol tap hole.
I considered filling it with vinegar and leaving it
but instead I decided upon electrolysis. I used an old
socket spanner suspended in the filler hole for an anode and
the tank as the cathode. The tank was filled to the
brim with a solution of washing soda and left for a
day. I did however taking the anode out and clean it
every two hours or so. After cleaning the current
flowing was around 4 amps (from a 12V supply). It
dropped to around 0.2 amps within around 15 minutes though.
When I emptied it after 24 hours, lots of rust flakes came
out with the water - just what I wanted. A blast
inside with a pressure hose released more, then the water
came out clean. I pointed a hair drier inside to dry it out
as much as I could then I quickly put the tap back, put the
petrol back in the tank and give it a good shake to remove
any residual water.
The outside of the tank could do with another coat of
paint. That's for another day.