Quoted from rai:Just curious, I've heard the term planking but what exactly is it?
Cracks in the artwork that run along the wood grain. They happen when a PF is exposed to moisture / humidity for a while, and the wood expands / contracts in between the grains.
For a closeup example:
http://s1337.photobucket.com/user/Retrophono/media/DSCF9445.jpg.html