Ghosting occurs when you sand or rub through one layer of finish into the one below, as shown in the accompanying picture.
You can recognize ghosting when the problem area you’re trying to remove keeps getting bigger rather than smaller—like sanding through veneer.
The term ghosting is the traditional name for this phenomenon. As it starts to appear, you see the “ghost” of the finish layer underneath. It is also called “layering,” which describes the phenomenon well, and “witness lines,” a relatively new term, which doesn’t. Nevertheless, it seems that witness lines has become the favored term in many recent wood-finishing articles and books.
Ghosting doesn’t occur with shellac and lacquer finishes because each coat dissolves into the previous one so that all coats become one. Dissolving doesn’t happen with varnishes, including polyurethane varnish, or with most water-based and catalyzed finishes. The separate coats form separate layers that are vulnerable to ghosting.
Sometimes you can disguise ghosting by rubbing with an abrasive such as steel wool. The problem is still there, but the scratches hide it.
The better solution is to apply another coat of finish after you have removed all the problems that caused you to sand deep in the first place. Then level and rub out this new coat without going through it.