In this picture from the factory files the title given is 'Anti Rust Treatment'. The picture is dated as in previous jottings and is, as one would expect, the next process as the shell moves towards the paint shop.
Today we tend to think of the preservation of the bare metal being accomplished by complete dipping of the shell in a series of tanks finishing with a primed or even fully painted body. Back in the 1950s and 60s the technology was just being developed and proven. Most preparation was still down to manual processes. A company such as Armstrong Siddeley were still delighting in the term 'hand crafted' and bearing in mind the relatively low volume of cars produced this made sense.
Research informs us that some form of phosphate based product was most likely used but one wishes the label on that can was legible!