This is a topic that has been discussed very feverishly online. You can normally get good information from various sources, but we feel as though it is too scattered even if Milk The Funk has a great Wiki on this. As such, this will be mainly for our reference and anyone else whom comes across this article.
We find the easiest way to check for leaks and to fix them is to Fill the barrel with 180F degree filtered water and determine if there is a need for repairs. While this Filtered water is in the barrel we then use this same temperature water and fill the barrel head. (flip the barrel on its end). Letting this water sit on the head until we no longer see any bubbles coming from the air escaping out of the wood. (then this is repeated for the second side)
Let the water inside swell the barrel until all leaks stop (if they exist)
After it is determined there are no leaks in the barrel we usually prefer to add whatever solution was in there before back into the barrel. (Whiskey Barrel we add whiskey, etc) We add about 1.5L of liquid per 5 gallons. this refreshes the interior of the barrel and will kill off any extra microbes that we do not want in there. We spin this barrel every so often (over the course of a day) as to coat the insides.
Now that we have some solution inside it is usually good to protect the barrel and reduce oxygenation by waxing the outside (Parafin Wax). The surface area of barrel to amount of liquid is much greater in the smaller barrels than your typical 59 gallon sizes. Waxing the barrel will emulate aging the liquid inside the larger barrels as it reduces the surface area exposed to oxygen.
A good rule of thumb:
5L (1.3 Gallon Barrels): Wax the entire barrel including barrel heads
20L (~5 Gallon Barrels): Wax the entire barrel excluding the barrel heads
40L (~10 Gallon Barrels): Wax 75% of the barrel excluding the barrel heads
60L (~15 Gallon Barrels): Wax 50% of the barrel excluding the heads
Process: For larger barrels 40L and up, it may be easier to use a crock pot to melt the wax and apply it with a paintbrush. Otherwise, we find that using an inexpensive travel iron works well. Setting the iron to the lowest setting and touching the wax to the iron will melt it slowly without burning it. Let the wax drop onto the barrel and then smooth it out with the iron. (Always keep moving as you do not want to burn the wood). This process is similar to waxing snowboards and skis.
This will be expanded as we find fixes to leaking staves, heads, or loose barrel bands. Currently we have been rather fortunate to not have a leaky barrel that required drastic measures to fix.
Use 1 pound of Barolkleen per 5 gallons to remove any existing liquid or trub in the barrel. Let this soak for 3 days. Use 180F degree water. (You may soak it for less time. We have had success when only soaking for 24 hours)
Rinse the barrel 3 times with cold water.
Use .3g / Liter of Potassium or Sodium Metabisulfite and 1g / Liter of Citric Acid. This translates into 6g / 5 gallons (20L) of Metabisulfite and 20g of Citric Acid / 5 gallons (20L).
This should be added to filtered water and the barrel should be filled.
Empty the barrel prior to use and give it one last good rinse before filling.
It is also good practice to replace the bungs on the barrel you receive. Some maybe silicon (which you may reuse, but the Oak Bungs received are usually in less than desirable quality. Plus if you plan on refermenting in the barrel you would like to get a drilled bung/stopper for your airlock. from our findings in the barrels we have gotten your typical sizing is below
5L (1.3 Gallon Barrels): Size #2 for a New Oak Barrel. Size #000 if removing the spigot and installing a port as described below in (Bottom Transfers)
20L (~5 Gallon Barrels): Size #5 for Spirit Barrels (Balcones) and #3 or #4 for New Oak Barrels (like in this link)
40L (~10 Gallon Barrels): Size #11 for a Spirit Barrel
Transferring To/From Barrel
At these smaller sizes and for the cost efficiency of the average homebrewer a Barrel Bulldog (Counter Pressure Filler) is not the most viable option. Instead we resort to using 1/4 SS MFL x 1/4 Barb fittings (Link). Drilling a small 3/16" hole at the bottom of one of the heads and installing this port with allow for bottom CO2 closed transfers. (this helps prevent oxidizing the liquid you plan to age). Lightly tap the MFL with a hammer for a snug fit.
If you find that the hole you drilled was too big. It is good to have some food grade silicon on hand to make a watertight seal (Link).
We then cap the port with a 1/4" Flare cap (Link). You can find these much cheaper on other websites than Amazon. (Google: 1/4" JIC Flare Cap) Make sure to get Stainless Steel.
Once you have transferred and capped the port check for leaks. If it is leaking it will eventually stop due to counter pressure or you can install a flare gasket.
Another helpful item to have on hand is a Flare Needle Valve. This makes it possible to transfer to and from barrels without much spillage when disconnecting and connecting hoses. It is a hard piece of hardware to find, but this site has them in stock if given a lead time of 7-10 days.
You may now use this port to close transfer from your fermenter or from your keg that holds your liquid. Consider this an advanced Vinnie Nail that you can also take samples from as your liquid ages.
If your barrel comes with a spigot/blank hole in the head of the barrel (instead of being flat) it is advised to purchase a smaller drilled bung and silicon it into place. In this drilled bung you can then insert a 1/4 SS MFL x 3/8" Barb to provide the same port.
NOTE: We have only used this closed transfer methods for sizes up to 10 gallons with success pushing with 2 PSI from a glass carboy. Bigger Sizes may create difficulties as the weight of the liquid being transferred to the barrel could theoretically be too much for 2 PSI to handle (DO NOT TRANSFER FROM A GLASS CARBOY AT GREATER PRESSURES IT WILL EXPLODE)
If you find that you are having trouble pushing it from carboy to larger format barrels. We would suggest you first transfer it to a purged keg. The keg can hold higher pressures much more safely. It it a lot more cleaning? Yes. Will you lose an eye? No. That is a fair trade.
Prior to filling your barrel with your brew and after cleaning the barrel with BarolKleen. Using a sanitizing solution of Sodium Metabisulfite and Citric Acid is wise. This step is not normally necessary if using a freshly dumped spirit barrel (cleaning prior is also not recommended) This is because the spirit barrel has had 40%+ ABV liquid in it prior so any bacteria in the barrel is more than likely (~95%) dead.
To sanitize, a solution of 1oz (28g) Citric Acid to 2oz (56g) Sodium Metabisulfite can be combined with 2 gallons of filtered water if using a 5 gallon barrel. Scale this liquid up per the size of the barrel you are sanitizing. (i.e. a 10 gallon barrel would be double this)
Slosh this solution in the barrel and let it sit for around 20 min. Shaking periodically (bigger sized barrels may prove difficult to agitate). Take care as to not breath in these fumes. I am unsure if they are toxic, but it is not the most pleasant experience.
Dump this solution out and fill the barrel completely with filtered water as this will be your rinse and CO2 purge liquid. Once the rinse solution has been purged with CO2 fill your barrel with your brew.
Length of Aging
Even though, it varies from person to person. This is a rough outline of how long to have your brew in a barrel to get proper flavor. Remember: It is always best to taste test your barrel aged brew periodically before kegging so that it doesn't have too much barrel character for your palate. This is also dependent on the style of beer you brewed for the barrel. A bigger ABV beer (i.e. Imperial Stout) will do better with more contact time than a lighter style. (it is always recommended to start with bigger styles in your barrels prior to stepping it down in SRM and ABV)
5L (1.3 Gallon Barrels): At this size these barrels have more surface area in contact with the liquid (Volume to Barrel Surface area ratio) as such they require the least amount of time to age. They also usually only come in brand new oak format so unless you are aging your own spirit in them then contact time is minimal.
First Use: About a week is all that is needed to get enough oak in your beer
Second Use: 14 Days to 18 Days
Third Use: 4 Weeks to 6 Weeks
Fourth Use: Most if not all of the character of the barrel is gone. At this point we turn it into a home for our house sour culture.
20L (~5 Gallon Barrels): This size has less contact area with the barrel. So it will require a slight bit longer aging period. Whiskey barrels impart a lot of flavor incredibly quickly, while new oak barrels impart flavor slower.
First Use: 9 Days to 14 Days
Second Use: 3.5 weeks to 5 weeks
Third Use: 8 weeks to 10 weeks
Fourth Use: Most if not all of the character of the barrel is gone. At this point go ahead and start doing barrel sours. You may still get some character from the barrel, but it will be minimal. Use Barolkleen and soak it for a few days to help remove the rest.
40L (~10 Gallon Barrels):
First Use: 15 Days to 21 Days
**Second Use and beyond is still being tested.**