Quoted from kwietsch:

some information to this overlay:
it was printed on a clear film and has an protection layer on top.
if you want to stick it on the playfield, you first have to brush it white.
this helps to make the printed colour to be more brilliant. after brushing
the playfield needs some layers of clearcoat to get a smooth surface.
to prevent damaging the overlay it is necessary to cut out all areas for
posts and similar stuff.
heinz