Put on rubber gloves to keep your skin protected from the drying effects of trisodium phosphate, also known as TSP.
Pour 1 quart of hot water into a bucket. Add 1 tablespoon of TSP. Stir the TSP into the water with a paint stirrer until dissolved.
Dip a clean sponge into the TSP solution. Scrub the grape juice stain on the surface of your porcelain sink.
Rinse the sponge as the stain is removed. Apply more TSP solution and continue to scrub to remove as much of the juice stain as possible.
Wash away the TSP solution by rinsing the porcelain sink with water.
Remove remaining grape juice stains with a solution made from 1 quart of warm water and 1 tablespoon of powdered bleach. Scrub the porcelain with a sponge soaked with the solution until the remaining stain is gone.
Rinse the porcelain sink with water. Dry the surface with clean cloths.