I realize that without providing sources, my comments may not be taken as any more reliable than comments here to the contrary. Below are some sources that may help clear some things up as to what’s really going on and may dispel some myths about all casks filled with scotch first being thoroughly washed out, dried out, smoked out, coaled out, etc. Enjoy!
1999 interview (it’s on the web) with then head of distilleries and maturation at Glenmorangie:
How many fills do you get from a cask?
My requirement is that the cask must not have been sitting around and be all dried out. Ideally the wine is emptied out completely, the cask resealed and shipped to Scotland within a couple of weeks. Most arrive dry but they do have a lining of crystals of tartrate and such like. The fortified wine casks are dry to moist but none of our casks has any lees swilling about in it. If that was the case you could ask if the flavour was coming from that but there must be some wood maturation for success. Any wine in the cask would be strictly against the law and the rules of the Scotch Whisky Association as it would potentially be considered as an additive.
From a scotch blog:
Whisky Magazine Issue 52 on 30/11/2005:
Whisky Magazine Issue 34 on 5/10/2003 (article mostly about Macallan):
When no longer appropriate for ageing malt, certain second fill Macallan casks are utilised as marrying vessels (being essentially inert with a minimal wood extractive influence).
A recharred bourbon cask, for example, could contribute less vanilla, coconut and sweetness, though a similar oakeyness and colour compared to a first fill.
Rejuvenated casks may hold up to two further fillings, after which there is the option of additional rejuvenation, with two rejuvenation treatments usually the limit.
This also raises the issue of what role recharred casks can play in the overall inventory. As the practise of recharring is only about 15 years old, research into the influence of recharred casks is still on-going, with the current focus being to age whisky for blends.
Whether rejuvenated casks play a more prominent role remains to be seen.