Quoted from ForceFlow:
As far as I was aware, that wasn't planking--just cracks in the topcoat due to age.
Planking usually shows as straight lines of raised paint. Basically, the wood grains swell, and push the paint off the surface of the playfield.
Yep. Planking shows lines that follow the woodgrain from wood expansion, usually from environment factors.
The spider web cracking that you see on older games before clearcoat/Diamond Plate are from the lacquer wearing out, and dirt gets into all the little cracks and lines creating the visible wear that you see. Only way to clean that out is with an abrasive (like Magic Eraser), at which point you have removed the lacquer and are starting to get into the paint, so you need a new top surface to protect it, either mylar or clearcoat.