nice testing Nino, and as you noted that it appears to have a mist coat in the corner suggesting the CC should of been at a much higher quality, but BB made a interesting point on the initial cooling as when cooling metal after heating can affect the molecule structure so it may play a factor as well with CC and I still believe that the ply is also a factor but the only way to determine that is by doing a Brinell / Janka Hardness Test. Quite often many quality plys are made with what is considered a clear timber = meaning no knots in the visual layers but who knows if there are some in the middle layers though I am sure if people had a poke around we would see those in any through holes in a playfield so pretty sure they are using select clear timber on all layers. The other is is slow growth (original trees) and fast growth (plantation trees) timber with fast being softer in strength due to the growing conditions and gone are the years where the timber is dried properly like the old days. Even on the CPR page they have removed the plywood info as they are looking at a new supplier. They state they prefer 7 ply over the cheaper 5 ply as 5ply fails quicker and some of the older quality playfields used 9 ply which is your bees knees timber probably from long growth forests not plantation forests. Lastly another factor is the type and quality of the glue used to bind each layer adds to the strength.

So unless someone takes a new playfield, a 10 years old one and 20 year old playfield and do a slice and dice of each one and run tests like Janka on a section with a mild clear, a thick stern clear, a HRP clear, a ceramic clear it is hard to determine the real failing point as to what is really happening:
- ply layers
- ply grade (old growth / plantation growth)
- glue used
- clear coat (mist & mild, thick, automotive and ceramic)
- curing conditions

In relation to clears, since I don't know anything I tried googling some info and say with the products at this link, various products have mixing recommendations, drying recommendations and all state various levels of being gloss, durable and flexible to a degree but are they truely designed for a pinball environment for impacts hence the CC generally holds up to the steel ball racing around a playfield, bouncing etc but feel the surface below is what is giving way to the impact of the steel ball. If the ply is softer and the CC flexible the CC adjusts in shape with the surface below (timber) denting.

http://www.hichem.com.au/hicheminfo/System%20Books/2%20PACK%20booklet.pdf


it would be a great test to do the above of a number of playfields and clears etc to cancel out some of the theories but pointless as their are alot of unknowns and doubt certain companies would ever share a study like this if even they have considered the various components (which I doubt), so the best we can do is at least ensure a decent CC is applied and then just enjoy playing the game and if you can afford it buy a spare playfield if you know a game is going to be your grail / keeper pin.