We use about 20 coats of Clear Gloss Oil-based polyurethane. Brushing is very time-consuming and we have recently discovered a new product from Minwax, which is a wipe-on poly. This is really nice and we are wiping on using squares cut from a white T-shirt, tossing each with one coat. If you do use standard brushing, the main thing is to clean the brush well between coats. We have set up a row of 4 cottage cheese containers of Mineral Spirits, with the last one being the cleanest. In the past, we also made a couple of drums with satin finish and then added paste wax. This is not as dramatic looking, but is a finish that holds up to the pounding if you intend to play the sides of the barrel in your songs.
In addition, we put 1-2 coats of polyurethane inside the barrel to protect it from deterioration over time.