I'm doing some repairs on a wood-trimmed composite canoe, and finding once again that the oil finish was applied by the original builder only after final assembly. Of course, the rot started in all the unseen places, but it hasn't gotten out of hand just yet. The canoe is only two years old, though. I'm thinking about oiling all surfaces, to get that tiny bit of actual absorption into the wood, then several coats of spar urethane, to provide better overall protection, since the owner isn't going to do any maintenance himself. Is there a mildewcide I can add to the oil, so it gets into the base layers?