I tried polishing the dented area with wire wool and cleaning it, revealing what looked like a pin hole, then soldering all over it (Oh how we laughed as the cylinder foam insulation caught fire, set off the smoke alarm etc etc). But there was still a leak coming from somewhere, so I decided the best long term fix was to change the cylinder. It's quite hard to find every pin-hole when the whole cylinder is encased in foam. In a similar situation I might try your idea of epoxy. Though, as you say, the key is to get everything really clean, which isn't always as easy as it sounds.