23 May 2015

Lambretta 150D Fuel Tank Rust Removal

A word of caution: Using phosphoric acid with high strength or sprung steel may cause hydrogen embrittlement. This could result in an unexpected failure / fracture of that part.

Working with phosphoric acid requires protective equipment and multiple safety considerations.



The inside of this Lambretta fuel tank was in poor condition with sixty year old two stroke oil, dirt and a considerable amount of rust scale. The photo below is what came out of the tank. Fortunately its shape makes it easy to clean (tunnelled motorcycle tanks are a little trickier).

The major constituent in some rust removal products is phosphoric acid. Look at the dilution percentage on the label when choosing, stronger is better. After cleaning out as much as possible with white spirits and plugging the petcock boss the tank was placed in a bund. This is a wise precaution as the phosphoric acid needs to be in the tank for hours and if it leaked can be quite destructive.

The phosphoric acid is added to the tank neat and begins a chemical conversion of the rust. The process takes some time, the Lambretta tank took multiple sessions totalling 21 hours. At best what will be left is most of the original metal with no rust, at worst there will be pitting and possibly pinholes in the tank. Before attempting this process consideration must be given to this being a possible result.

The example below is a BMW tank where the knee pads are attached. The rust has been removed but in this case the depth of the rust was too deep and had to be cut out and replaced.

A few stubborn patches of rust / contamination remained in the tank which were removed by adding handfuls of building screws and water, wrapping the tank in old towels and tumbling it in a cement mixer for about an hour.

Stopping Flash Rust

The last crucial step stops or reduces flash rust on the inside of the tank. After a final phosphoric acid treatment, immediately use methylated spirits to wash out the tank. Methylated spirits absorbs the residual water in the phosphoric acid thereby preventing flash rust. Allow to air dry and the methylated spirits will evaporate. Be careful not to allow methylated spirits to come into contact with paint, it may damage it.

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