Sprinkle scouring powder into the cast-iron sink. Add a small amount of water and scrub the sink clean with a scouring pad.
Rinse away the scouring powder with water and dry the sink with a clean towel.
Sand the sink with 220-grit sandpaper if the sink has flaking paint. Wipe away any debris or dust with a rag. If the finish is in good condition, skip this step.
Apply an oil-based enamel paint to the cast-iron sink with a paintbrush. Let the paint dry and apply a second coat.
Wet a piece of 600-grit wet/dry sandpaper under running water and add a few drops of dish soap to the sandpaper. Sand the dry enamel paint with the wet sandpaper to create a smooth and shiny finish. Wipe away the suds created during sanding with a damp cloth when you are finished.