Almost all clearcoats require cure time, usually somewhere from 2 weeks to a month to fully cure. If you build up the bike too soon after clearcoating and have several clamp-on components (front derailleur, downtube shifters/cable stop, etc), you can cause the finish to crack and flake.

That said, if you're going to Death Valley you probably aren't going to need to worry about your bike's finish too much. ...Well, at least as far as it pertains to rain or moisture. I'd just as soon build it up and ride it for the Death Valley tour, then clear coat it when you're done and have the time.