There are several products out there that can be found at any local hardware stores to fix small chips in porcelain. Most of them are a type of paint applied with a medicine dropper type applicator or a small brush similar to fingernail polish. If you can find the right color, fingernail polish is a quick fix for small chips. This will be more of a “band-aid” type repair, though–and won’t stand the test of time and use. The real porcelain repair products are applied and, once dried, need to be buffed off to a smooth finish. If the chips are large or too many to fix, you can hire a professional porcelain restoration company, and they can make your tub or sink look new or better. Of course, this is the more expensive option, so if your budget doesn’t allow it, we recommend trying a DIY fix first.
Info from: Randy Ritzenthaler