Rusty fuel tanks are a frigging nightmare. Loose rust can cause blocked taps, clogged jets and engine damage. Sadly, many of the solutions are also a pain. In a bid to find an easier solution, Sticky mixes up an acid solution.
I can’t remember where I first read about oxalic acid as a rust remover but I immediately liked the sound of it. There are lots of chemical fuel tank lining solutions available, all of which seem to be a pain to apply and a massive nightmare when they go wrong. Those who lined their tanks with old-school Petseal may now find that it’s all flaked and fallen off due to the higher ethanol content of modern fuels (the latest Petseal Ultra is designed to withstand ethanol). If your tank sealer comes loose then it’s worse than the rust.
With Oxalic acid I found a potential rust solution that might avoid the need to seal the tank at all. Or at least that was what I hoped.
Oxalic Acid – what do I need to know?
- You can order as a white powder from eBay and get it delivered through the post.
- It’s commonly used in low concentrations as a ship decking, wood bleach or driveway cleaner. Instructions for such are usually supplied on the packaging.
- It’s a mid-strength acid so take sensible precautions in use (rubber gloves, eye protection, don’t put it on your dinner etc etc).
- It eats rust and forms a water soluble salt leaving metal clean.
- Amazingly it has almost little or no effect on paintwork.
- It’s as dangerous a chemical as household bleach, so read these MSDS Safety instructions before use. Keep animals and kids away from it.
How to use?
I’ve twice used Oxalic now as a cleaner for rusty petrol tanks, but there are some provisos to its use.
- Firstly, it is good at eating rust, but it won’t perform miracles. If your tank is full of loose rust flakes that constantly block the fuel tap then it may be beyond repair using acid alone. The tank may need sealing, in which case you need a modern ethanol-resistant tank sealer such as Por 15.
- Secondly, it can only eat the rust if it can get to it. If your tank is full of varnish or old oil where old fuel has evaporated then you might need to get that out first so the acid can reach the metal.
Stage 1 – cleaning
The method I chose to use was to use detergent to try to remove any old residue. You could start with sloshing a litre of fresh petrol around but I went straight for the “half a bottle of washing up liquid and a jet-wash to agitate” method. If you really want to do a proper job then you could rig up some form of rotating box affair and fill the petrol tank with pebbles, ball bearings or old nuts and bolts. Given that the most recent tank I had to do was a Vega one; which has a tube inside the neck meaning you couldn’t get anything like that back out again, the jet-wash method would have to do.
If after draining the tank there are still loose particles, then you need to go through the washing process again.
Stage 2 – take the acid
If you are cleaning loose parts you can do so in a plastic bucket, but if it is a fuel tank then you can simply fill the tank.
You need to seal the tank at the outlet (re-fitting the tap should be fine) and then fill the tank to the brim with a strong mixture of oxalic acid and hot (or freshly-boiled) water in a ventilated environment.
The recommended water to Oxalic acid mixing ratio for rust removal is 10:1, so for 10 litres of water add 1kg of acid crystals.
Stage 3 – wait
In experiments with this process I was amazed how fast you get results; even 20 minutes shows a clear reduction to surface rust. Obviously the longer you leave the acid (and the stronger the concentration) the more rust it will eat.
You can feel the acid working from the outside of the tank as it seems to stay hot much longer than you’d expect if you just filled it with hot water.
After you are happy with the results (the longest I’ve left anything is overnight) you can drain the solution. Oxalic acid is bio-degradable.
Stage 4 – rinse and dry
With acid you have two choices to neutralise its effects. You either use an alkali – such as bicarbonate of soda – in water to neutralise, or you simply rinse and rinse until the acid is so diluted that it has no effect.
I’ve tried both methods. The problem with using bicarbonate is knowing what ratio to use and also making sure you get it out in time. On the Vega tank we recently treated, the bicarbonate mix was drained out but we forgot to dry the tank. By the morning the insides were coated in surface rust again. Doh!
On that basis, I’d therefore suggest that simply rinsing the parts in a great deal of clean water is probably sufficient, but the most important issue is to get it all dry again afterwards so that rust doesn’t form on the fresh metal surface.
Stage 5 – after-treatment
After drying, assess the quality of the job. If you still have loose rust then you either need to repeat the whole process or try a tank sealing method.
Once dry, you can use a mixture of petrol and 2-stroke oil to coat the inside surface of the tank.
This is the point at which I’m probably going to get criticism from those in favour of fuel tank sealants.
In both cases I haven’t bothered to seal the tanks. When they were new they were bare metal and the mixture of fuel and oil (for a 2-stroke) sloshing around inside was enough to protect the steel. Rust will only form when air and moisture can attack the surface, but if the scooter is regularly ridden then it should be fine without lining.
I can say this with some confidence because one of the fuel tanks in the photographs is a Maicoletta tank that we cut open and treated with Oxalic acid in 2013; before converting the scooter to run a 400cc Suzuki engine for our Frankenstein Scooters trip to Istanbul. With a 4-stroke engine we’ve not put oil in the tank, only petrol, but it has been fine and the rust has not returned in four years.
If you were using this method to remove rust from a chromed part, then you probably need some form of barrier like a polish or ACF-50 to prevent moisture from attacking the chrome and causing rust to re-form.
VIDEO: setting up a fuel tank filter rig
Stage 6 – filtering
In my experience, even after sealing a fuel tank with a commercial product, it is wise to use some form of filter on the fuel line to catch any loose particles before they enter the carburettor. After welding up the Maico tank I set up a looped system with a fuel pump and a paper-type fuel filter that fed back to the neck of the tank and left that running for a day or so, swapping filters until they remained visibly clean.
Alternatively you could just buy a normal fuel filter in-line to prevent residue from reaching the carb while riding, but one thing to consider is that paper-type fuel filters are designed to run with pumps and if you use one on a gravity-feed system then it will slow down the flow considerably which can lead to fuel starvation problems.
While Oxalic acid is not an infallible method to clean an old, rusty fuel tank, it certainly works where the contamination or corrosion is not too severe.