How to treat rust in your tank with Phosphoric acid

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How to treat rust in your tank with Phosphoric acid

Post by popkat »

Sorry no pictures but hopefully this is good enough for you to give it a try.
My tank had surface rust and the results were very good. Phosphoric acid leaves a fine coating which protects from flash rusting which will give time to fully dry the tank and fill it with fresh fuel, it should also make the work last a good time with no reason for the tank to rust again if stored correctly.. My advice would be if you've used this treatment is when storing the bike up to fill the tank with fuel, ideally super unleaded and also ideally with a fuel additive such as Forte Moto 1&2.. Put the additive in on your last ride so it gets through the fuel system them fill up and store, this will stop any rust forming and also keep your fuel good over winter...

Phosphoric acid shouldn't damage your paint but if you get some on the paintwork wipe it off quickly with a wet rag just in case.

I kept mine after the treatment as it's good for cleaning anything rusty (although it leaves a whitish finish to bare metal) and I could also clean another tank if filtered first. Other uses are as concrete cleaner, drain cleaner and it's also found in foods, tooth paste and coke amongst other things, much much more diluted of course. If you don't want to keep it then water it down a lot and pour down the drain.


I used phosphoric acid from this place ... SwBahVSdOd this one is 25ltr but I ordered just a 5 litre bottle, they've not got it listed at the moment but worth asking them if they have some.

I also got a cork bung from here which was 100mm iirc ... 2749.l2754 again it's not currently listed but give them a shout they may have one.

My tank had surface rust not thick rust, it was still getting through to the carbs though. I also got a new filter as my one came out damaged, they aren't cheap (about £30) so save yours if you can. I drained the tank and once empty I left it for any last bits of fuel to evaporate. then I cut the cork to make 2 bungs, I bunged the filler cap and fuel filter/tap holes, I cut the bungs close to size then put them on the grinder to make the shape, this bit is a bit messy and dusty so wear a mask. if you have loose rust in the tank put some water in and sleush (is that a word ?) it around to get as much out as possible, then when empty put you bungs in and seal them with some silicone sealer. Leave over night to go off.. the next thing I did was to de solder the wires off a spare fuel sender unit, this is the hole I put the phosphoric acid in, I might not have needed to do it but as I had a spare I didn't want to risk messing up my fuel sender. you could just de solder then re solder after it's only two wires, I also unbolted the plunger so all I had was a plate with large rubber O ring.
Tank will now be ready.

I poured 4 litres of water in the tank.. THIS BIT IS IMPORTANT, ADD THE ACID TO THE WATER NOT THE OTHER WAY ROUND !!!!!.. ... acid.shtml
then I slowly added the 5 litres of acid to the water.
Bolt on the fuel sender plate.. if you secured the bungs properly you will have no leaks, would be worth a quick leak test first with water if your not sure..

I first let it sit for about half hour then rotated onto it's side, then top then other side, I gave all sides/angles half hour then turned it upside down took the sender plate off and had a look, I think I gave it about another quarter of an hour on each side then was happy with the result.. just keep checking but I don't think it needs hours and hours, well mine didn't... remember the acid I used was 81% before diluting.

Next I removed the plate and drained back into the original bottle and two 2 litre plastic milk bottles. then took the tank outside put it on a couple of old sheets and hosed it out, spent about an hour doing it to make sure no traces of acid were left.. Note if you have a dirty path do it over that, the acid will clean it right up but it will notice (phosphoric is also concrete cleaner). once clean I drained the water out as best I could, dried tank off and put a hair dryer in the filler hole. it is important to fully dry the tank, although phosphoric leaves a fine coat mine had a couple of rust colour streaks next morning where I ran out of time to fully dry it, this was not rust just discolouration and very slight. Other than that my tank was like new inside, I was very pleased with the result.

I then put the tank back on the bike and filled right up with fuel, I wasn't using the bike straight away so I put super unleaded in as there's less or no ethanol, but you could fill with regular and add a fuel additive such as Forte Moto 1 and 2 to the fuel and give it a gentle shake to mix it in. even if you don't it's worth using the stuff once a year as it does work well..
I have had no problems with my tank or the way the bike runs since I did this.

Lastly if your fuel tank is rusty take your carbs off and give them a good clean, there's guides on here for removing/refitting them, be careful with the chokes and don't mix the jets up.

2014 CRMC Post classic Superbike champion.
2014 CRMC Post classic senior production champion. On a Suzuki Katana 1100
My bikes, Firestorm, Yamaha MT10SP, 2 x Honda RC24 road and race, Popup Katana and a some XJ600's

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