Andrew,

All depends on what era the machine playfield is, and what condition its in.

Magic erasers strip layers of clear / paint off.  So if the playfield is worn, you can make it worse.

Shadow and Demo man are both 'Diamond plate' which effectively is form of 2pac clear coat (basically automotive clear).

Now, as playfields were made by different manufacturers back in the day, screened and cleared by different manufacturers in the day, the levels & amounts of clear vary.  Some even used a different clear to Diamond plate...

Interesting read on the history here :----> http://www.flippers.be/pinball_playfields.html

Everyone has different methods, different products they swear by so its a bit like saying who makes the best beer or pizza.

For me, I have cleaned a few DMD (Williams / Bally) over the years.  It all comes down to what you start with and experience.

Any playfield clean begins with a full teardown of all topside components.  These are cleaned by hand, polished, tumbled etc.

All underside components are also removed and cleaned.  I always replace coil sleeves and other consumables at the same time.

Now, I generally clean the black grime / dirt with nifty on a rag.  Never spray directly on artwork or the playfield.

If the playfield is dull (the same as the duco on a car) it may need to be cut and polished.  Many use the 'Treasure Cove kits' purchased from overseas which do a great job of this (albeit messy).  A quick pure carnubra (non liquid) wax and polish and you're done.

If the playfield still has some shine to it, a wax is all I give it.

Novus is effectively a cutting agent, which does a good job.  The treasure cove kits I believe are a more automotive grade product, which have been developed / modified for pinball playfields.

Novus is - as far as I know, a product designed to cut and polish plastics and similar products.