Next on the list - rust spots on the inside of the tank.
Brief sidetrack - I wanted to see more of the inside of the tank so spalshed out £9.99 on a Eurocarp parts endoscope. Don't, not sure you even get what you're paying for. The software supplied simply doesn't work. Tried a number of apps, a random USB camera one sort of did.
It showed a lot of the tank with these:
Someone had recommended DEOX -
https://www.bilthamber.com/
So I ordered some DEOX and some Surfit degreaser. Washed the tank out with degreaser and then mixed up the DEOX and poured it in. Through the filler cap you could see a patch of the tank with a light coating of surface rust, the DEOX got rid of that just pouring it in
Left it for a couple of hours, swilling it around every 15 mins or so. The results are pretty impressive where the metal was submerged, slightly less so where it was only getting wetted when swilled as you'd expect.
Submerged
Swilled
Overall I'm happy there's no rust to wash off and block fuel filter or rust through the tank.