Many shells already come with some kind of mild oil or even a factory applied thin urethane coating on the inside. This is to protect the shells interior exposed wood from humidity, temp changes, etc. Cheaper shells are pretty janky on the inside, so maybe there's something to adding a coating to cheap shells, but damn, polyurethane? I used to coat my skateboard ramps with polyurethane. Heavy stuff. I don't think I'd want that inside my drum shells, but whatever. To each his own. I just wanna know what he's trying to get out of it. Maybe there's a less aggressive way to do it.