Commercial rust converters vary in their chemical makeup, and generally aim to convert the iron oxide (rust) to a black oxide that is inert and can't harm the tool further. They often incorporate other polymers that then dry on the surface of the tool to form a protective layer. This changes the finish of the tool, and if you wish to restore the steel back to bare metal, will need to be removed by mechanical means. This property also makes rust converters hard to use on the flat backs of secateurs, as it can add thickness that prevents the blade and the anvil from sliding over each other. Rust converters can be useful in removing the last bit of rust after mechanical removal or electrolysis. If you are determined to use one, try it in an area away from the blade edges first to see how it responds.