A wooden bowl exterior can be tarred only as a decorative or outdoor craft finish, keeping tar away from the inside of the bowl and any surface that will touch food.
Tar gives a wooden bowl a dark, old-looking exterior, but it is not a food-contact finish. It belongs only on the outside of a decorative bowl, reenactment prop, display piece, or outdoor object where staining and smell are acceptable. The inside should remain untreated with tar if the bowl will ever hold food, drink, herbs, or anything handled near the mouth.
The bowl exterior should be dry and finished to its final shape before tar is applied. Use a very small amount on a cloth or brush and rub it into the outside surface. Carved lines and foot rings can collect dark material quickly, so keep the coat thin and wipe away heavy buildup before it hardens or stays sticky.
Let the bowl air for a long time before display, especially if it will sit indoors. Tar smell can linger and may transfer to shelves, cloth, and paper. A tarred exterior is best treated as a historical or rustic visual effect, not as a general bowl finish.