Here's a set of my Typhoons. Have a close look. Clearly on their last leg. Glued and sealed in 2008, they are still solidly on, yet starting to peel themselves from the base tape. This is an example of how water...when it seeps in and under the sidewall sealant (AquaSeal in this case) damages the cotton and wreaks havoc on the cotton.
Note that these sidewalls were sealed after the tires were glued on. I will be applying sealant before I glue this year, trying a new product alongside AquaSeal as well to determine which works best. Much like FMB or Challenge treat their products. Dugasts are sealed, yet I'm not sure with what! It simply can not stand up to the elements.