You will need to fill it with water for a week, I'd say, just to make the wood rehydrate and inflate. 99% chance that it will leak for a while, so do it in the garage or something like that. The wood will inflate and it will stop leaking, and now you're good to go.
If it doesn't stop leaking, you can fix it with wax (capping wax used in wive bottling, or bee's wax)
For that big, boiling water is a point... you will need imagination!
But anyways, here how I do for my little oak casks. From what I've been told.
So now your barrel is full of water, and you are ready to sanitize it. Next steps are:
- Empty it
- Fill it about half of boiling water, then shake it for a while to get rid of any wood debrits inside (some put a metal chain inside to scrape the inside surface)
- Empty, then refill with boiling water and Sodium Metabisulfite (1 tbs / gallon)
- Let sit for 24h, turn/roll it regularly
- Empty, rinse good 3 times.
- Refill it with clean water, let it sit for another 24h.
- Empty, and you're good to go.
Always keep full of water when not used.