Reading beer history, one notes that in the days before stainless steel brewers would assiduously avoid the leaching of wood flavor into their beers. They would line their barrels with various substances, pitch being one. Wood was the vessel, not the flavoring. (Many exceptions, of course, such as sour beers, staled beers, etc.)
But it's the opposite now.
Brewers today — especially smaller scale, so-called 'craft brewers— will often court and deliberately encourage the flavor of oak. Since no US breweries have paid coopers on staff, they purchase used their barrels from bourbon or other whisk(e)y distillers. Beers aged in these oak barrels (53 US gallons) will gain a spirits flavor (and possibly even some alcohol).
Then, there's the issue of spoilage. Bacteria and yeast are lurking everywhere; they adore the crevices of wooden barrels and any sugars they might find within.
I asked the past production manager of a large mid-Atlantic brewery (now closed) what he and his brewers had done to care for and 'feed' their wooden barrels.
At *******, we always used fresh barrels, so there was no need for much care or sanitizing.
In a 'fresh' bourbon barrel (just recently emptied) the staves would be moist and the bung still in place holding some residual pressure.
If a barrel had dried out, we would rehydrate it with water. It might take a few days for the staves to swell up, so we would just kept topping off until the barrel sealed.
On the occasion when we did reuse barrels, we would rinse and fill with hot water for cleaning and sanitizing.
We tried to keep things simple at *******.
And final use?
Pushing up pansies.