I've been having good luck using tent floor seam sealer on the skinwall portion of older tires that would otherwise be too brittle to ride. You do have to know the difference between tires just needing an appearance boost and those that are too old/damaged to try this on, though.

Makes the sidewalls supple again and prevents fraying. Again, I'm not sure I'd depend on it for long rides in terms of restoring durability of the sidewalls. However, doing this on a set of marginal tires before they get "spiderwebbing" seems to keep them looking fresh and rideable.