Any chance of a Photo not 100% sure what you mean by Planking?

I think rob is referring to the effect of the wood expanding due to moisture, and causing lines to appear down the playfield. Very common on machines I've seen with predominantly black playfields. Waxing does seem to improve the appearance, to the point of "filling" the cracks somewhat and I can't see it doing any damage. I wax "planked" playfields.