I wonder whether there is a difference between a new machine that is assembled and shipped to Australia for example and is opened up and played as soon as the new owner gets it and a machine that might sit on the shelf for a few months before being bought i.e. the clearcoat is given more time to cure whilst it is sitting on the shelf.  Would this be a possible explanation as to why some playfields dimple quite badly and others haven't?

My Metallica LE is wearing pretty good for a Stern machine and doesn't appear to have as much dimpling as other Stern machines I have had.  It wasn't opened and played for at least 6 months, probably more like 9 months I think.  The magnet in front of Sparky has significant dimples and I need to remove it every few months to file down the burred edges to stop it damaging the playfield - it is very soft metal but all in all the actual playfield isn't too bad which seems to support the theory that maybe a longer cure time is needed.