General condition of a watch is important.

If the watch is in a particularly bad state, then it maybe time to sell them a new one. The older cheap watches used a nickle "base metal" alloy for the casing which is eaten away by the customer's sweat leaving gaping holes for dirt and water ingress. In some instances, the cases are so bad that doing a good job is actually a bad idea. The process of cleaning dirt from the watch case, which is normally essential for the integrity of the watch, removes the only thing protecting it- the dirt. If the customer insists on "repairing" it, put the battery in, hope for the best and then wash your hands.