Stubborn rust stains can form on a porcelain sink, particularly from a shaving cream can or other such metal item sitting on the edge and leaving a circle of rust. Even bleach, commercial abrasives and hydrogen peroxide can fail to get the stain out. And many homeowners are adverse to using a product caustic enough to require gloves to use. The trick is to find a substance harder than rust but softer than porcelain. That substance is pumice stone.
Purchase a pumice stone in the skin care section of a drug store.
Wet the stone and the rust with water. Using a dry stone could scratch the porcelain.
Scrape the iron with the pumice stone directly, as if confining a pencil eraser to just the pencil markings you want to erase. This concentrates your effort and reduces the chance of hurting the porcelain. The stain should come off easily.
Wet the pumice stone with oxalic acid if it does not work on its own. Use gloves for this.
Apply car wax paste or Windex to the porcelain to restore its shine.