Apply a generous quantity of product with a brush or a spatula to the area affected by the rust, leaving a thick product layer on the stain.
Allow the product to react with the stain, checking now and then if the stain has been removed, by rinsing well with clear water.
Possibly re-apply the product, should the result be unsatisfactory ( a specific interval of application time cannot be recommended, as few minutes or many contact hours, up to a whole night could be necessary to remove the stains).
Check the result. If the stain persists on the dry surface, but less intense, repeat the application.