Here is a NOS Stern Tron playfield.  This was a clear coating only job.  On some Stern playfields the upper layer of plywood is not as hard as the American Hardwoods brand that other companies have used.  This softer wood WILL dimple.  Clear coating these Stern playfields will NOT prevent dimpling.  Urethane clear coat flexes (just like it does on your vehicle bumpers) so if the wood is soft it will still dimple.  Tron playfields are known to dimple.