Lawrence our Quality Controller comments, “Depending on the age and use of the cask, the liquid progressively soaks through the depth of the stave and it is not uncommon to see it almost halfway through the stave on older casks. The “soaking through” process is extremely slow due to the grain orientation of the wood (for wood enthusiasts- staves are quarter sawn meaning the medullary rays – shown as lines in the cross-section photo below – are mostly parallel with the stave face, making them non-porous). Indeed, if the staves showed soaking of whisky further than say halfway through, the cask is nearing the end of its life and would probably start to leak way before there was any danger of contact/soaking with the paint surface.
Cask heads are much thicker than staves; an average of 50% thicker. He continues, “Any paint used for cask ends would only affect a few mm at the surface (see picture below) and it is fairly inert once dried. In any case, the wood effectively isolates the paint from the whisky within. Hope this helps…”
Incidentally, Lawrence has been known to salvage discarded casks for creative projects, and adds, “Removing the paint is usually the hardest task..! I generally find a few strokes with an old rasp plane gets it off quite well, then I finish up to a neat surface with a fine set No.4 plane.”
Thanks to Lawrence for the photos: L a cross section showing the medullary rays running laterally through the stave; R the split cask head showing the veneer of flaking paint and no signs of deeper incursion.