Playfields and Clearcoat
I am trying to understand the failure modes that we see in modern clearcoated
playfields, and figure out what we can do about it to restore and preserve our
machines. Thanks to Bryan Kelly and KarlZona for some of the pictures and
insights, and to Cliffy for showing me how to whack a raised insert.
The Problems
Here are the five problems that I have identified so far:
Lifting inserts - common in many machines, but clearcoats have a special
attribute because of the curing shrinkage. Example: Funhouse clock
Dished Inserts - Larger inserts seem to dish below the playfield surface.
Example: Attack from Mars
Separated Clearcoat - milky areas over inserts. Example: Cirqus Voltaire and
Tales of the Arabian Nights
Cracked Clearcoat - around inserts. AfM and ST:TNG are prime examples.
Inclusions - tiny defects in the clearcoat cause by bubbles and particles.
Working Hypothesis
My working hypothesis is based on statements from KarlZona and Mirco,and
supported by some googling, that
clearcoat continues to shrink after application and initial cure.
As the clearcoat is attached to the playfield surface, it cannot contract in
area. Therefore, it has to become a little thinner. It must also apply a tensile
force to the playfield surface.
Although the plywood is too thick and stiff to move, the same is not true of
the inserts. They are made from relatively thin (polystyrene?) material. The tensile
stress in the clearcoat causes them to dish downwards. This is especially true
of larger inserts.
If an insert is not fully locked down, this force can cause the insert to
lift (see Funhouse example below).
If the insert was not properly prepared, or the surface was contaminated, the
clearcoat may separate from the insert, leaving a milky appearance. Common on
Cirqus Voltaire and ToTAN.
If the clearcoat did not flow into the gap between the wood and the insert,
it may crack across the gap.
Is AfM a Special Case?
AfM playfields, old and new, seem to be susceptible to cracking around the
inserts. The cracking happens around large and small inserts, so it is a general
problem. My working hypothesis is that the AfM CAD file made the holes a few
thousandths too large, leaving a gap that the clearcoat did not penetrate. This
gap results in cracking over time.
Fixes
1) All inserts need to be locked in place. Water thin CA on any suspect
areas.
2) Raised inserts can be dropped back in place with a block and hammer. Don't be
scared.
3) Inclusions can be block sanded, spot cleared and buffed. Not usually
worthwhile.
Funhouse playfield after the clock center insert had been leveled (with
a hammer and block). The insert had risen by about .02" at the top edge.
The
underside of the insert, now locked in place with CA adhesive. 
A
small inclusion in a funhouse playfield. 
A
small inclusion in an AfM playfield. Note the fine bubbles in the
clearcoat, not visible under normal conditions. 
Stern
(Churchill) insert showing full contact adhesive.
New
reproduction AfM
playfield showing spontaneous clearcoat cracking, attributed to
clearcoat shrinkage and failure to fill gap. AfM's fast balls will seek
and destroy any vulnerable inserts.
Here's
a test from KarlZona showing how a sanding sealer makes the inserts look
diffuse (the two circles on the right, the one with teh blue marker has
the sanding sealer). Without a sanding sealer, the inserts are crystal
clear. This also happens if the pre-ink coat is not a sanding sealer,
but some other form of clear.