The most effective way of removing rust is by scrubbing with Qxalic acid which is commonly sold as “Rust and Stain Remover”. Oxalic acid is also the basis of most proprietary timber deck cleaners.
Oxalic acid is commonly found in various plants such as the green leaves of rhubarb. You will note most brands of stainless steel cutlery warn against leaving the cutlery in rhubarb! (As an interesting aside the Italian name for the deadly nightshade plant – also containing Oxalic acid – was Bella Donna meaning “beautiful woman”. This was because of its use to make women more sexually alluring. Women would crush the leaves of the plant in their fingers then wipe their eyes. The Oxalic acid would act to enlarge the pupils!)
For rust removal you mix 100g of Oxalic acid INTO one litre of COLD water (never use hot water and never add water into the acid) and scrub. Oxalic acid reacts with iron hence its use despite being a hazardous product. The same ratio was the basis for Rusteban which was sold to remove rust stains from clothing. You can buy a product identical to Rusteban as K2R Rust and Stain remover.
Another, less hazardous, product used to remove rust stains is CLR.
Prolonged exposure to Oxalic acid can be dangerous and it is skin absorbed.