Well if you want to go to the trouble you could always spray on a clear coat of enamel afterwards. If you order a short run of CD-Rs from
Short Run Music, that's exactly what they do. But you could do it yourself. I made some alarm stickers a few years ago, and the company that made the sticker paper I used said, that if you wanted to place these stickers on the outside of glass where the rain would get to it, you'd need to spray it with a caoting of enamel. They recommened using Rust-OLeum Crystal Clear Enamel. It also has the added bonus of giving your CD a glossy shine to it.
It might be kind of a pain to do though. I think I'm gonna try Short Run Music for my next CD and let them do it. CDBaby recommends them and I haven't heard anything bad about them yet, so I guess I'm gonna give em a go.