Cleaning steel tanks

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Cleaning steel tanks

Post by Hugz »

Haven't tried this method yet but sounds as though it has merit. Found on a BSA site:

Petrol Tank Cleaning and De-Rusting

Les Howard recommends a four stage process for stripping, cleaning, de-rusting and drying the petrol tank on your classic motorcycle. Appropriate clothing should be worn...

One of the worst chores that is so often required on a classic bike today is renovating the inside of a steel petrol tank, usually because of rust or an old tank sealing lining turning to a sticky goo. I have found a method and chemicals that can make the job at least a little easier, if not a lot easier, with minimum expense and great results. It's basically the same process as most commercial products would utilise but not necessarily the same chemicals. It works out to be far cheaper than buying all the branded stuff and in my experience works incredibly well.

If you are the type of person who is accident prone, careless, frightened of using strong chemicals or going to ignore safety warnings then it would be better for yourself to ignore the method I am going to describe! Leave the tank for someone else to clean out or carry on with the same method that you already use.

I advise wearing eye protection, rubber gloves, a plastic apron, etc. I definitely insist that at all stages you have a large bucket filled with cold, clean water, even a water hose, ready to use, beside you at all times should you accidentally splash yourself anywhere with the chemicals. Your first reaction must be to wash off chemical that has somehow got onto your skin or spilt elsewhere.

So at this stage I've probably scared off everyone from attempting this method as it sounds like it's massively dangerous. So long as you act confidently and carefully it is not inherently dangerous and mostly no different to any chemical tank cleaning method. I usually do the work in my carpet slippers, T-shirt and no gloves without any problems, but I know I am taking a slight risk and perform the task with extra care. Just be careful.

The two main chemicals used are caustic soda (sodium hydroxide, sold as a drain cleaner) powder or granules, and spirits of salt (hydrochloric acid, also sold as a drain cleaner). As an extra, phosphoric acid can be bought to finish the job if required or desired.

Caustic soda is dead cheap and 500g is enough. It can be bought in hardware or DIY stores or eBay. Spirits of salt is also dead cheap, can be obtained similarly. Phosphoric acid: at least 40% strength is required. Plenty on eBay too.

Stage 1: Gunge Removal

To start, pour out all the remaining petrol from the tank into a can and if it's still any good save it for use at another time or in your lawnmower. I like to remove the fuel tap filters and re-fit the taps (making sure they are switched off) as they survive the whole process pretty well intact. If not, you should fit bolts to block off the holes. NEVER fit corks or putty or similar goo in the tap holes as this will come straight out and leave you saturated in chemicals.

I use the existing petrol cap which is usually enormously rusted too and it's going to look like a new one (inside) as a nice bonus when the job is completed. You MUST then immediately tape over the cap vent hole with duct tape. It sticks very well or you can use a self tapping screw (but this could still leak, I've found). If you forget to block the cap hole, then again you are going to get strong chemicals all over yourself so do it right at the start as it is easy to forget.

The next stage is to place the tank squarely down somewhere outside or on a garage floor. Fill the tank to within about three inches from the top with clean water. Then with all the safety clothing on, slowly pour the entire contents (500g) of the caustic soda crystals into the water. This will raise the water level and also heat the water up quite a bit.

NEVER pour the powder into the tank and then add water. This is extremely dangerous and hazardous as the soda will boil the water this way and create a small explosion of steam and caustic soda.

After adding crystals to the water, top up the water level to the neck of the filler opening, covering the upper inside surface of the tank. After the liquid has cooled down a bit, the tank cap should be fitted extremely tightly and left to soak. Leave for several hours, then return suitably clad and rock the tank around a little. Carefully and slowly release any build-up of gas from the tank by slightly unscrewing the cap just a touch. Then re-fit the cap and rock the tank from end to end to fully mix the soda and water and disperse evenly.

Leave the tank for several more hours and then check it isn't leaking water. If it is you are in a bit of trouble. If the leak is very small you can place the tank in a large outer vessel to hold the leak, bearing in mind that the chemical will strip paint very effectively which might not matter at all if you are renovating the paint job later.

If it's not leaking, then simply leave to soak for another day and return again for further agitation and rocking. How many days will it be best to leave it? Well I have safely left tanks like this for about five days to get the full benefit but definitely two days to get good value and cleaning power.

When the time is up, you will have to dispose of the used soda and water. You will have to do this regardless of whatever chemical you use, so follow the recommended environmental method. Please note that this chemical solution will be still extremely strong and powerful and will not have been slaked or weakened by very much at all. It will literally dissolve skin -- so be very careful.

(It is worth noting that caustic soda is used in abundance to clean grease from household and industrial drains. Knowing this might give an insight to which method of disposal can be viewed as OK!)

At the end of stage one all the old petrol gunge, any previous tank sealant and quite a bit of rust will be softened, removed and flushed out with the solution. The colour and filth of the liquid which leaves the tank will amaze and inspire you to go on to the next cleaning stage; to finish this stage simply hose out tank several times with a strong jet of fresh water.

Stage 2: Rust Removal

Now we have the rust removal to do and this is the bit that I get impressed by most. Spirits of Salt is a concentrated (Fuming strength) hydrochloric acid. I use it normally for chemically cleaning toilet bowls where there has been a build up of lime scale. This is what it is sold for mainly. It is very powerful and works quickly and effectively. So as a household product, it should not be too frightening, but I must emphasise it is very, very strong and because it fumes it will choke you if you inhale the fumes. Sounds dead scary, but just be very careful, no need to become terrified.

Pour about a pint of clean water into the tank and then carefully and SLOWLY add about a third of the 500g of acid, trying to avoid inhaling the fumes. NEVER pour in the acid and then add the water for the very same reason I mentioned before with the Soda, ALWAYS Acid ONTO water.

Screw back the cap and just like before, do a slight rock of the tank and then equalise the internal pressure by loosening the fuel cap and then tightening back on firmly. Next, you have just a bit more work to do than last time as it will require the tank to be rocked, shaken and inverted to completely swill around the small amount of acid so it can work on every part of the inside areas of the tank. Do this for a few minutes, and then leave for ten minutes. Return and carefully remove the cap. By checking with a torch, you will now see how well the acid is cleaning; you will also see how that rusty tank cap is starting to look quite healthy again on the inside.

You may now add another third of the acid and continue the agitation as before and leave another ten minutes. At no time will you need to rock the tank for more than a minute so there is not much effort required if this is worrying you. Once again re-inspect the inside with the help of a bright light. At this stage you should have quite a bit of gleaming un-rusted metal tank looking at best quite like it did at manufacture. I know it's hard to believe but it will. Further acid strengthening and agitation will eventually, but still quite quickly, remove all the rust or burn it away.

I have found that NO abrasive components like shingle or bolts need to be added to bash off the rust flakes…hard to believe isn't it? But no, the rust flakes will just have disappeared, possibly leaving the inside as clean as the proverbial whistle. Once you are at a level of cleaning that you're happy with, stop the process and dispose of the acid solution, noting once again the acid is used for drain cleaning! Hose out and shake the tank thoroughly several times quite vigorously as the acid will have got into seams and will continue to etch away at the metal if left un-rinsed.

Stage 3: Nearly Finished, Etching

You now have the tank with the inside cleaner than you would have thought possible a few days earlier. You'll be pleased of course and you might simply just want to rinse out very thoroughly and go to the drying stage. Another possibility is that you might consider neutralising any possible acid with an alkali such as a small touch of household bleach and rinse again and then go to the drying stage, but whatever you choose, get to the drying stage as quickly as possible as the metal inside is likely to start rusting slowly again if left damp.

I suggest going to a further acid stage using a quick splash around of Phosphoric Acid. This type of acid is not likely to etch the metal away strongly and will create a slight surface protection if a very slight amount is not washed completely away at the final rinse. So after about 10 minutes, stop the Phosphoric acid stage and pour away this relatively weak acid. (Not quite as corrosive to yourself as the Spirits of Salt in fact it is an ingredient of fizzy drinks). One quick water rinse is all that is needed now to finish. Another thing to note is that phosphoric acid usually converts brown rust flakes to a black compound but if you have done stage 2 to a fully bright clean result, there will be no brown flakes of rust any way to convert but the bright steel can be etched with some benefit of the anti corrosion resistant skin it can produce.

Stage 4: Drying

All we have to do now is to dry out the water as quickly as possible to ward off the spoiling of the nice metal finish by rust. Shake out as much water as you can, and I find there is a point that just flipping the tank quickly but quite lightly around with the filler-hole at the lowest point is the most effective as the water flips over the slight internal edge of the hole better this way. I know this sounds obvious but it's difficult to describe.

I then use a hot air gun with the tank taps removed to get some air circulation through the tank. I stand the tank vertical so it acts like a chimney and aids the hot air movement, taking out the damp air with it. As the tank heats up quickly, it is better to try to rock the tank around at stages as the remaining water tends to puddle in crevices and when moved and spread quickly over the hot metal, it evaporates very quickly. Doing it this way, I can get a tank bone dry in about 15 minutes. Don't heat the tank continually, by the way, with the hot air gun as you might do some damage to it. Always check underneath the filler cap which will be the hottest part.

You will now have a clean fresh tank inside and you have your own choice as to whether you go to a further stage and apply a tank sealing compound. I'm not going to describe that process here, but it's going to be dead easy as the hard work has all been done. My last tank, a genuine un-restored 60 year old Norton tank cleaned up so well I am very tempted to just stop at this point and use it without further sealing protection but I'll have to see and decide later.

During the drying process, you might just get a very tiny amount of tarnishing of the metal but nothing to worry about as sealing compounds will bond to this quite firmly and strongly.


The method uses very cheap but very strong, easily available chemicals.
Physically quite easy, not much agitation of the tank required.
Outstandingly clean results, far better than one could expect.
No nuts and bolts or gravel required! Well I didn't find any were required.
Cleans your petrol cap up inside like a new one.

Finally no need to rush anything but don't leave the Spirits of Salt stage too long just in case in etches right though the metal. I have not seen a massive corrosion taking place and if the tank is going to leak it is better to have the acid clear away the weak metal to leave strong metal edges for sealing to.

Just be careful and act with concentration and with safety in mind at all times. I also don't want to scare anybody off and hope I have not over emphasised the "dangers" as I would be prepared to do all the work naked in my kitchen without fear of damaging either me or the kitchen. Perhaps that thought is the scariest of them all, so I'll get dressed now...
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Re: Cleaning steel tanks

Post by Collector Inspector »

I would not follow this advise Hugo.

Insane to use such methods dealing with all of those acids. This is something from the 70s.

Salt and/or vinegar as well as the old 10:1 water to molasses remedy is the way to go.

Use molasses option........original paint intact.



C still going strong by the way
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Re: Cleaning steel tanks

Post by Oyster 49 »

I have used caustic/pea shingle in my bike tank, followed by phosphoric acid. It worked very well, removing some surface rust, but not deep seated.
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Re: Cleaning steel tanks

Post by Chris B »

Regarding the perceived insanity of using 70s methods...

Playing Devil's Advocate for a moment, it could be argued that if old and potentially injurious technology is a reason for not doing something, then perhaps we should all steer clear of British Seagull outboard motors.

So having pondered that point for a moment or two, let's move on and have a look at the de-rusting of steel tanks...

Over the years I've refurbished a number of pressed steel fuel tanks. Two belonged to vintage motorcycles, one was from a portable generator and two of the more recent were the extra long range Seagull tanks. And having tried various methods of corrosion removal, the technique I eventually came to favour for small steel tanks is reverse electrolysis. And I favour it because in terms of results it's proved to be head and shoulders above the various alternatives.

For the benefit of the uninitiated, reverse electrolysis is an electrochemical process which, in this context, uses low voltage electrical current flowing through an electrically conductive solution to deposit new metal on the inner surface of a corroded tank. When set up carefully and used properly, the process strips away rust and scale and simultaneously deposits a very thin layer of fresh steel on the inside of the tank. One of the advantages of reverse electrolysis for tank refurbishments is that, unlike methods that use corrosive liquids to strip metal away from an already thin tank wall (which is a bit counterproductive if you think about it) the RE process transfers metal TO the inside of the tank instead of removing it.

Here are the upsides... First: everything required for treating a tank using the reverse electrolysis method is likely to be in your home already, or can be bought inexpensively at your local supermarket and a DIY store. Second: after treatment, the inside of the tank is spotlessly clean and the newly deposited metal (albeit only a few microns in thickness) has a uniform pale grey colour with a semi-matt texture. Third: the resulting finish is perfect for lining with POR15 or whatever else you prefer to use to protect the inside of the refurbished tank.

And now the downsides... The trade-off against the advantages and efficiency of reverse electrolyis is that there are risks attached. The electrolyte is a powerful alkaline solution, the electrochemical reaction generates hydrogen and - if you're not meticulous about detail and tend to be even ever-so-slightly casual about being careful - the process can result in electrical sparks flying. Sparks in the vicinity of hydrogen are obviously best avoided, and with the foregoing in mind I'm going to avoid making this too easy, so I'm not going to provide the actual details of DIY reverse electrolysis here.

I'm just informing anyone who didn't already know, that there is a well proven method for refurbishing seriously rusted steel tanks, and superb results can be achieved inexpensively in a home garage or workshop. But please - before you explore the subject of reverse electrolysis any further - be aware that care and attention to detail are vital and that getting it wrong can have serious consequences. However, and to put the perceived risk and the associated caveat into some sort of perspective, so can driving to the supermarket, or getting tangled up in a Seagull flywheel.

Last edited by Chris B on Fri Sep 20, 2019 3:06 am, edited 1 time in total.
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Re: Cleaning steel tanks

Post by haventaclue »

My preferred method is taking my heavy duty 12v battery,a bucket/basin of water,a s/s eating utensile and a couple of spoonfuls of bread soda.
Stir bread soda into water. Attach wire to rusty object and place in water.
Attach wire to s/s implement,place in water.Make sure both do not touch.
Attach wire of eating utensil to pos. pole and wire of rusty object to neg. pole and leave for a couple of days.Rust pops. Wash with water hose,dry off.
This method does give of a gas,but if in well ventilated area,you should be ok.
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Re: Cleaning steel tanks

Post by Chris B »

Haventaclue is on the right track and his method will work - but a far better result can be achieved by using sodium hydroxide (NaOH) instead of bread soda (aka baking soda). Sodium hydroxide is readily available over the counter in crystal or powder form and it's sold as "washing soda". The electrolyte solution you mix up will require approximately 4 heaped tablespoons of soda crystals per gallon of water. The ratio isn't critical but if you use too little of the soda then the reaction will be sluggish. Use gloves (and preferably eye protection too) when handling the soda and mixing the electrolyte. When you're done, wash the spoon thoroughly and dispose of the gloves.

If you're treating a mild steel tank then your electrode needs to be the same material - so you need unplated mild steel rather than stainless. Apart from being incompatible with the reaction you need, stainless steel creates toxic by-products - and in this setup we've got enough of that going on already. Use a chunky piece of mild steel for the anode - a big heavy bolt, old spanner or something similar is ideal. Just make sure it's bare metal and not plated.

If you're going to treat the inside of a Seagull fuel tank then your preparations need to include removal of the Ewarts fuel tap and closing the vacant fuel port with a blanking plug.

Probably obvious but nonetheless worth mentioning that if you're treating an object other than the inside of a tank, then the bucket or basin you use to contain the electrolyte should be plastic or some other non-conductive material.

Once running, the reaction will cause the electrolyte to fizz audibly, producing the gas that haventaclue mentions in his post. Please be aware that the gas is hydrogen.

The treatment time will depend on how much rust needs removing' and also on the current that you feed to the circuit. In the case of heavy rust and surface scaling, the reaction might need to run for several days. If so, shut down the reaction every 24 hours or so and replace the electrolyte with a fresh mixture. At the same time, clean off the electrode which by now will be covered in thick, muddy looking crud which will be inhibiting the current flow. As before, don't forget to wear gloves and eye protection.

A longer treatment time using a low current (3 - 4 amps at 12 volts) produces a better result than using a high current for a short time. Also, if you use too much current when de-rusting a tank, the electrolyte will get a bit excited, frothing up and overflowing all over the place. Far better to go at it gently and be patient. If you're treating a small tank - such as a Seagull fuel tank - I suggest placing it in an old baking tray, or an oil drip dray, to catch any fluid overflow. If your refurbishment doesn't include respraying the outside of the tank then spend a few minutes masking off the exterior carefully and thoroughly, so as to keep any electrolyte splashes or spillage away from the paintwork.

Incidentally, the effective circuit is "line of sight", so if parts of the object you're treating are hidden from the electrode then you'll need to either rotate the object, or move the electrode from time to time, so that the electrode gets to "see" the entire object. This line of sight issue tends not to arise when you're derusting the inside of a tank, unless the construction uses folded and welded joint seams with hidden gulleys in them.

If you decide to use a battery charger rather than a battery for a power supply, you'll need to use a simple, bog standard, old fashioned charger. A modern "smart" charger's microprocessor isn't programmed for this job and it won't know what to do with the load that it's seeing - so will usually refuse to play the game. Not so smart after all...

Plenty more detailed and useful information on this subject can be found on websites dealing with classic motorbike restoration. If you want to try this method then I strongly suggest that you have a look at a few of them before you begin. Steel fuel tanks on old bikes are always a problem and the foregoing method is a favourite professional way of refurbishing them.

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Re: Cleaning steel tanks

Post by dandrews »

I think we are getting our sodas muddled up here.
Caustic Soda = Sodium Hydroxide, a hazardous chemical
Washing Soda = Sodium Carbonate, a mostly harmless soapy cleaning solution
Baking Soda = Sodium Bi-Carbonate, a food ingredient and raising agent
Ice Cream Soda = a fizzy drink

Substituting any of these for another will end in tears, at best.
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Re: Cleaning steel tanks

Post by Chris B »

Dandrews - thanks for the correction.

Although sodium hydroxide (NaOH) and sodium carbonate (Na2CO3) are both used in electrolyte solutions, sodium carbonate - which, as mentioned in my earlier post, is sold over the counter as “washing soda” - is the more appropriate choice for DIY projects.

We regularly used sodium hydroxide solution for reverse electrolysis corrosion treatment in my marine engineering workshop, but it’s more aggressive than sodium carbonate, and whilst careless spillage or splashes of either can cause unpleasant skin burns, sodium hydroxide will generally inflict more damage, more quickly, to skin tissue.

Regarding accidentally using one instead of the other: for the application under discussion NaOH and Na2CO3 are interchangeable. Either can be used for the electrolyte solution but - because they behave differently - it's necessary to know which one you're dealing with!


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Re: Cleaning steel tanks

Post by seagull101 »

I boiled vinegar and used that to clean my Honda SS125A tank but was unhappy of the result, it did remove some rust but not enough for me to be confident that no rust will enter the carb. Although not the safest it sounds like this "70's" idea is the best method to use.

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Re: Cleaning steel tanks

Post by Charles uk »

Reverse electrolysis, means the finished item is the anode, unlike normal electroplating where the copper is the anode (in the case of copper plating) & the item that is being plated is the cathode.

Reverse electrolysis in this case using a caustic solution, would cause anything that is soluble in the caustic bath i.e, ferric oxide, any plated finish on the tank inner wall & finally iron to be removed from the anode (tank inner surface) & carried over to the cathode where it would probably fall to the bottom of the tank when it loses it's charge as this solution is not suitable for electroplating, the grey finish on the tank insides is where the surface iron has been dissolved.

Many years ago I worked for Vanderval Products in the Lab, responsible for the technical side of the plating shop, where they electroplated Lead, Indium & Tin on the wearing surface of crankshaft bearings & bushes, all of the bearings were cleaned prior to plating, by reverse electrolysis in caustic baths.
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