Fuel tank rust treatment with phosphoric acid

So you have found rust in your fuel tank.  I found evidence of fuel tank rust in my fuel filter when diagnosing why fuel pressure was decreasing over time.  Left untreated, fuel tank rust can cause more problems than low fuel pressure. It is also worth noting that with modern fuels using more and more ethanol that the risk of fuel tank rust increases particularly with older tanks.

So what can you do about the rust?  There are a number of ways to treat rust.  Here I have chosen to use phosphoric acid as a fuel tank rust treatment.  If you’d prefer to watch a video then head to the bottom for a youtube video or click here

Convert the rust?

One of the simplest ways to treat rust and prevent further rusting is to convert rust into an inert material which also doubles as a protective ‘cap’.  Phosphoric acid converts Iron Oxide into Iron Phosphate.  This also renders the rust inert.

Is phosphoric acid suitlable for all rusty fuel tanks?

Not every rusty fuel tank is a candidate for treatment with phosphoric acid.  The rust in my tank was light surface rust all located on the bottom lower areas of the tank.  If the rust is heavy and the remaining steel is thinning, using an acid might result in formation of holes in the metal.

The process

There are three steps to using phosphoric as a fuel tank rust treatment which are prepare, use and clean up.

1. Prepare

Removing loose rust and debris as well as degreasing the interior of the fuel tank is key.  To remove loose debris there may be benefit in adding a length of chain or couple of handfuls of aggregate to the tank, sealing the tank and shake the fuel tank sufficient to loosen the rust.

In my instance, the rust was limited to the surface so I cleaned the tank with carb cleaner and made sure as much debris was removed and I was set to go to the next step.

2. Apply phosphoric acid

The contact duration for the phosphoric acid will vary depending on the product. The acid I used was a specific rust converter product and recommended 15 to 30 minutes contact time.  I choose to leave the acid in contact for 30 minutes.

I used 1l litre of acid and ensured that the tank was positioned so that the acid had contact with all the rust in the tank.  I did chock the tank on the table to achieve appropriate contact.  Initially the acid did fizz and foam which is an indication of the chemical reaction taking place.

3. Rinse and clean

After 30 minutes of contact time I emptied the acid into a container for appropriate neutralisation and safe disposal.

My product recommended rinsing contact areas with methylated spirits which I did twice and both washes I captured for safe disposal as well.


So how did my fuel tank rust treatment fare? Pretty good in my opinion!  Not only was all the rust converted to Iron Phosphate but much of the fuel tank was cleaned and descaled by the acid.  I reckon some parts look like what the tank would have from new back in 1971.