One disadvantage to choosing a material such as porcelain for your kitchen sink is that porcelain can actually crack, which causes leaks and subsequently a wet mess in your kitchen. Sink repairs of any nature are difficult because sinks are hard to keep dry while you wait for the repairs to set. Since replacing a sink is expensive, you should take a little time and see if you can fix the crack in your old one first.
Dry your sink, and sand the cracked area of the sink until the edges are smooth.
Pour a small amount of rubbing alcohol on a clean rag, and wipe the sink to remove dust and grit from the sandpaper. Let dry.
Combine a small amount of high gloss paint and porcelain repair compound in the glass bowl. Mix well with a putty knife. You want to get the colour of this mixture as close to the colour of your sink as possible. Add more paint or compound as needed to adjust the colour.
Scoop up some of the compound mixture with the razor blade, and apply it to the crack in the sink. Keep adding more until the crack and chips are filled in completely. Remove any excess compound by scraping it off with the razor blade.
Allow the area to dry thoroughly. Soak one end of a cotton swap in nail polish remover, and use it to blend the edges of the compound into the porcelain.
If possible, don't use the sink for 24 hours to let the repair dry completely.