Rust stains can be accelerated by the presence of salt and the resulting stain will be much deeper and more stubborn to remove. Painting over a rust stain will stop it from growing because the iron is sealed off from its exposure to oxygen, but that isn't always an option.
For more stubborn rust stains, make a paste of lemon juice and laundry booster. Apply with a scrub sponge; scrub, then rinse. Toilet stains can be removed with this same paste. The laundry booster helps remove the stain and restore the sink or tub to its original appearance, just as it would remove a grass stain from a pair of jeans.