I just finished gluing my tubulars for this season's 'cross campaign and I had a quasi-cyclo-cross-related religious experience downstairs in my basement that I just had to share. I used a urethane sealant on the sidewalls of the tires called Seam Grip. It has been lauded on other forums, which is where I first learned of it. It's manufactured by McNett, the same company that makes Aqua Seal. The Seam Grip has a lower viscosity than the Aqua Seal, which means that it goes on easier and requires a much smaller quantity than the Aqua Seal. Also, it comes with a really handy applicator that is the same width of several of the more popular, handmade 'cross, tubulars. Plus, it also comes with a very small acid brush for touch-ups. I've tried Aqua Seal and I've tried Jevelot's Tire Life, which is a very-thin, latex-based sidewall protectant, and I've got to say, this Seam Grip business is the way to go... although, it could be that I'm a little "high" on the stuff from all of the fumes!
I don't have any financial ties to McNett or Seam Grip. Here's the website for the product: