My bowls are all finished with food safe materials, such as linseed oil, walnut oil, beeswax, and carnauba wax.  The finish seals the grain and protects the wood. To clean after use, wipe with a damp cloth as needed; never submerge a wooden bowl or put it in the dishwasher.  My experience shows that the finish will usually last a long time; however, with frequent use you may find that the finish needs to be touched up a bit, which can be done with mineral oil or walnut oil.  Do not use food oils as they will go rancid. A bit of beeswax will also restore the natural luster.