Rapid after-rusting is a problem with any of the chemical [including electrolytic] de-rusting methods.
One of the best ways to passivate the surface is to rinse it off thoroughly, and swab it with diluted Phosphoric Acid solution before it dries. This will form a thin protective grey coloured Iron Phosphate surface coating, which is also a good base for the primer coat. Nearly all of the commercial 'rust remover' products are Phosphoric Acid based - if it doesn't show on the label, the MSDS will say. Try a dilution of about 1+4 to start with; if the metal surface doesn't turn grey pretty quickly, add a bit more rust remover.
Thanks gadge :), just wondering, do you prefer getting straight Phosphoric acid and diluting if needed or do you recommend just getting a specific ready-to-use rust remover/converter?
Also, picked up a set of those twist knot angle grinder attachments, seemed to work quite well.. even on my finger :p .. took off a heap of rust from the blade plate.. primed it up, gonna give it a quick coat of black and chuck it back on the mower..
