Learning about the wood, the history, and the craftsmanship of a barrel in advance can save you troubles down the line. So, the first thing is to have a reputable [barrel] supplier, if you can, and to know as much about the supplier and barrel as possible. Ask questions, and you might be able to figure out how the barrel was kept, where it was stored, and what kind of use it got. But understand that working with older wood, there are always going to be risks.
It’s also important to know that not all oak or wood is the same and that the thickness or thinness of each stave is going to matter.
When I first look at a barrel, I check the hoops to make sure they are in place and tight. If the hoops are loose, you might have to steam or fill the barrel to get it to swell out again and fit the hoops, but this isn’t always ideal. If you paid for a bourbon barrel or sherry cask and you steam it or fill it with water, you’re really just washing away all that great essence that you wanted for your beer. So when possible, try to get barrels that you don’t have to steam. If you do have to steam or fill the barrel, just make sure the water isn’t above 160°F (71°C). The good news is that you don’t often see this problem for homebrewers unless, perhaps, you acquire barrels that were freebies or have been sitting around for a while and need to be steamed out. In that case, you can use an inexpensive wallpaper-removal steamer to do the job.
I also look for leaks and weeps when I look at a barrel. You should be able to see where things happened in the past, so pay attention to any stains or signs of previous repairs.
Tap on the staves to make sure they aren’t loose. If a few of them are, sometimes you can just knock the loose ones back in with a hammer.
If you fill your barrel with wort or beer and it starts to leak or weep, there are some fixes. You can purchase barrel wax, which you can essentially spoon out and cover any cracks or leaks. You can also make a mixture of flour and just a little water to create a paste, and that works really well on wood, almost like mortar or concrete. For small holes, you can get little pieces of wood to jam in, and I know some people even use golf tees.
For foeders or large-scale barrels, there aren’t a lot of options for dealing with leaks because of the size and the weight of the liquid pushing outward. There is a food-grade wet epoxy that you can put on the inside of the foeder or barrel when it’s empty, and that usually holds tight.
At the brewery, if we have a barrel problem, hopefully we have other barrels that we can transfer to in an emergency. If you’re a homebrewer, it might be good to have some spare carboys if you run into serious leaks.
After you’ve done this for a while, you can start to get a feel for a barrel and what is good or not. You can tell whether something isn’t right.