How to clean a rusty petrol tank using Oxalic acid

I cant see that this would work if your tank leaks and I dont know about if its been previously sealed either.

the bike was a gsx1100g with about a 3 gallon tank. it was very rusty as had been standing for about a year 3 quarters full of fuel.

I think I got the concentrations right so if you've a smaller tank, use half as much water / acid. oxalic acid occurs naturally in rhubarb and suchlike so its organic and greener than other crazy james bond acids that are available. Note that Oxalic acid is a lot stronger than Vinegar, which would probably do the job eventually .

drain the tank using a siphon. get it off the bike. remove the overflow pipe if you have one. keep the fuel tap on the bike. Plug the fuel and reserve pipe / overflow with a bit of pencil and some gaffa tape or what ever. i tried to use just tape however the boiling water / acid melted the glue and it came away.

nuts and bolts inside the tank tied together with a shoelace and about a half litre of stale petrol - I used the petrol I couldn;t get out via siphon. dont use nuts that are too big as you could dent the tank from the inside when you shake it. I used about 10 nuts about 2 inches long and bolts about the size of a hazelnut,

if you have a float inside the tank for your petrol gauge the nuts on a string will ineveitabley get tangled, so you may need to gain access to the tank, usually from the bottom. otherwise dont tie them, but difficult to get out as my tank has lip.

A good shake to remove the lose rust, then drain. dont go mad - you dont want to dent the tank. I wanted the nuts and bolts to drag along the rust to loosen it. I drained into a washin up bowl and had about 2mm of rusty crap come out that covered the basin bottom. so it was pretty rusty inside.

Remove the fuel tap, rinse the tank with a few litres of water to get as must rust particles as possible, drain. people will say dont use water in the tank - your rinsing - not leaving it for any period of time. get all the nuts and bolts out.

check you fuel tap and filter - the bit that goes in my tank is plastic and steel - not alloy or or aluminium - which may react with the acid. if your is alloy you'll need to find another method of sealing.

wrap a plastic bag around the fuel filler cap and close the tank. you dont want acid leaking out the breather holes. lie it on its top.

oxalic acid does not seem to react with paintwork, and I must have got some on myself at some point, but I did not have any burns, tho I wore marigolds. no point in temptin fate tho so be careful.

order a kg of oxalic acid of ebay - mines was about £2.50 with £8 postage cos its heavy.

boil the kettle, find a measuring jug, 2 heaped tablespoon of powder in the jug, topped up with about 3 quarters of a litre of water. it will fizz a bit, so take it easy when adding the water. give it a stir with a wooden spoon, add to the tank via the hole where the fuel tap was. I added about half the container to 5 litres of boiling water, then added another litre for the pot.

put the fuel filter back on, your tank should now be liquid tight.

give it a good shake around then lie it flat as if it were on the bike. acid likes to be warm - I kept my tank in the sun. I did most of this outside in the garden. my girlfriend does not appreciate motorbike bits, never mind acid, in the kitchen.

if your tank is rusty, you're probably doing you carbs too, so go off and start them. every time you pass the tank, give it a shake. lie it on its 4 sides and give it a shake. lie it on its perpendicular and give it a shake. I left mines for 8 hours. I shook it about once every 45 mins and left the tank in different positions to ensure the acid got into every nook and cranny for a decent amount of time. The rust will disolve into the mix. I was swooshing it around. be aware of any leaks. it may or may not be bad for your skin and other surfaces. I expect it is if you leave it for too long. Wipe up any spills. Have a filled watering can nearby just in case you need to wash some away or rinse it off yourself.

after about 6 hours, open the fuel tap, peek inside. should be quite a bit cleaner than it was. if not reseal, leave it longer, shake it about. Note the bag around your fuel tap will probably have a hole in it now due to the tank lock so you may need to replace. the acid does not (seem to) react with the plastic.

I experimented with some nuts and bolts left in a teaspoon of acid in 100ml of water in a jam jar for 9 then 24 hours. At 9 they were clean and rust free, at 24 the acid had started to eat into the bolts which oxidised and green scum developed. So I think 9 hours max is the length of time you want to leave it to avoid further problems with the tank surface. no point in removing rust only to have to get the scum out as later.

once your happy with the rust situation, drain the acid into a washing up bowl by removing the fuel filter. fuel filter should be rust free, as should the tank. acid will be green with brown bits flaoting in it. looks a bit like anti freeze.

thoroughly flush the tank through with water. some people recommend using borax or similar to neutralise the acid and stop reaction, but I didn't have any and dunno where to get it. I rinsed it 3 times, shaking it around, using 3 watering cans, then left it standing on its end fuel tap end down to drain. there will still be water in it - get as much out as you can by shakin the tank about.

refit the fuel filter, make sure its still blocked. tank should be liquid tight again.

buy a 500ml bottle of methylated spririts, add the lot to the tank, shake it about. meths absorbs water do when the meth evaporates so does the water leavin you tank water free.

unblock the reserve fuel filter pipe and drain the meths back into the bottle. no point in wasting it.

if you are really keen at this point you could seal the tank with POR 15 or all those other mixtures that may or may not work. My tank was spotless inside and the bike will be used daily so rust should not get a chance to come back.

open the fuel filler cap, leave to allow the meth remains to evaporate, tho it'll burn so this step may be obsolete.

refit tank and your set. you may have to remove tape glue from the fuel pipes where you sealed them with gaffa tape. wd40 gets it off.

i disposed of the acid by diluting with the watering can again and putting it down the drain.

comments welcome - this was a trial and error thing but a few people have posted similar successes elsewhere on the web

Andy