Seal the Inside of the Barrel
Fill the barrel to the brim with water.
Mark any leaking areas on the exterior of the barrel with chalk.
Empty the barrel of water and allow to dry.
Apply mastic to the corresponding interior sections of the barrel that match the marked locations of the leaks. Allow mastic to cure according to the manufacturer's specifications.
Apply a bitumen sealant to the inside of the water barrel using a paintbrush. Let dry for 24 to 48 hours.
Seal the Outside of the Barrel
Rough up the exterior of the wooden barrel with a medium-grit sandpaper to ensure proper adhesion of the water sealant.
Apply an even coat of the exterior wood sealant using a paintbrush.
Allow to dry for six to eight hours.
Even out drip marks on the first sealant finish using medium-grit sandpaper. Apply a second coat of exterior sealant. Allow to dry.
Sand out drip marks again and apply a third coat of sealant. Allow to dry.
Smooth the finish with fine-grit sandpaper.
Apply a final coat of exterior sealant.