We have an oak barrel for use as a planter for some wysteria on a stone patio. I would like to know what should be done to preserve it.
The barrel is an old rum barrel but not guaranteed to be watertight.
Advice seems to be mixed on whether they need to painted with varnish or sealant. I've put what I've learned so far into an answer in the hopes someone can improve on it.
Life expectencies seem to range from 2 to 20 years.
The advice below comes from researching on Google. I am not an expert so take it with a pinch of salt and please suggest corrections and clarifications as comments or edits.
If the barrel was used to store rum, wine or whisky it should be quite durable. The inside was probably charred to make it watertight. Decorative barrels sold as planters tend to be of lower quality. A real barrel was probably made of red oak rather than white oak which rots more rapidly.
The barrel probably failed a pressure test preventing it from being re-used or sold on for storing alcohol in the future that would suggest its not the best possible quality but it should still be good enough as a planter.
The barrel should be raised off the ground on stones, bricks or terracota plant feet so that the bottom doesn't rot.
You should drill drainage holes in the bottom. Consider covering them with mesh (to prevent insects crawling in?)
The barrel should be kept moist (water it when you water the plants) as if the wood dries out it can shrink eventually causing the rings to slip and the barrel to fall apart.
To protect against the rings slipping you can tack nails under the rings.
You should put a liner on at least the base (but with holes lined up with the drainage holes).
A liner on the inside may help prevent the soil drying out.
You might consider spraying the inside and outside with apple cider vinegar to inhibit growth of fungus.
Advice is mixed on whether you need to varnish it. Beware that if you do, the varnish will need to be re-applied periodically as it will fade gradually possibly giving a blotchy appearance.
Rust on the metal band can be cleaned off with vineger.
Many people recommend putting stones in the bottom of the barrel to improve drainage. There is little evidence this helps but it might save you on soil.