Get a 20L tank, stick 500g of citric in it, stir.
Really is as easy as that.
Citric acid is my go to situation. Bucket with hot water, some citric acid and away you go. Never measured anything. Always works
I tried some citric acid. Pretty disappointed. Electrolysis or evaporust for me now.
for bigger items use the wheelie bin after rinsing it out
I mixed some citric acid with boiled cornflour and water to make something that would stick like wallpaper paste to derust a radiator, worked very well, took some time but worked well.
Something like this added to the butt
https://www.ebay.co.uk/itm/Backer-7-11-18-Immersion-Heater-Hot-Water-Thermostat-Genuine-Part/181203895909?_trkparms=aid=888007&algo=DISC.MBE&ao=1&asc=20140328180637&meid=24f494b7aedf4d95bdbc4226dc614727&pid=100009&rk=1&rkt=1&sd=153140852316&itm=181203895909&_trksid=p2047675.c100009.m1982
I tried some citric acid. Pretty disappointed. Electrolysis or evaporust for me now.