OK. Before going on, let me make two points: (1) Stans is a good product that fills a unique need in mountain biking. I use Stans sealant. I like it. Our store sells it. (2) The company that sells Stans assures me that their product does not weaken rubber in any way.
But from personal experience with dozens of Stans-filled tires, and after hearing from many mountain bikers who use Stans, I'm convinced that non-sealing tire failure is an occasional complication of sealant. It may not be the fault of the sealant, but these failures occur in ways that the riders say they've never seen in tires that didn't contain Stans. The two most common types of failure are (1) sidewall bursts, which are more common in non-UST tires filled with Stans, and (2) de-lamination (bubble formation) leading to non-sealing blowouts, more common in UST tires.
So what's going on? I think it's a combination of tire-manufacture defects (such as poor adherence of the outer rubber to the cords, or areas of inadequate rubber coverage on the inside of the cords) interacting with something in the sealant. Possibly ammonia gas, I don't know.
New front and rear UST tires of same model, both filled with an identical quantity of Stans from the same bottle, at the same time.
At one year's time, the front tire is rock-solid and never loses a molecule of air.
At age 2 months, the rear wheel occasionally showed these damp-looking spots after riding. The tire would loose air, from 32 pounds down to 20 over about two weeks' time.
With time, the air loss became more rapid. At age 10 months, the tire could lose up to 5-10 pounds overnight.
Ten months after the original tire mounting, inspection revealed a damp interior with white fibrous material but no free-flowing sealant. 60 ml of fresh Stans was added. After riding 10 miles and sitting overnight, the tire again again showed a couple of "wet spots" and developed subtle bulges.
Further, the tiny "ooze" spots (which are probably tiny punctures) didn't correspond to the bulges.
What I think would have happened: When the weakened area fully ruptured during a bike ride, this tire would have suddenly spewed sealant and deflated completely. And that's something I've experienced before, maybe 8 times over 6 years of heavy riding and racing tubeless.
(1) Age of tire. Older tires accumulate stresses (such as the flexing of cords in the sidewall of an underinflated tire) that may expose the cords to the liquid sealant or cause the outer rubber to separate from the cord layer. Even a chemically inert substance such as water could contribute to tire failure if it soaks into the cords.
(2) Length of time exposed to sealant. Using one brand of non-UST tires, I typically experienced sidewall rupture after 3 month's use. But also, I've had the experience of placing fresh sealant in a tire that's already had Stans for several months and had the tire fail dramatically on the next ride.
(3) Brand of tire. Certain brands, and I'm not naming them here because I don't relish hearing from lawyers, have been reported again and again. But even within a specific brand, manufacturing anomalies may affect only a certain year's model, or tires coming from a specific factory.
(4) Non-UST. The thicker rubber in a UST tire tends to protect against all types of failure, sealant or no sealant.
(5) Bum luck. As in the example above, two supposedly identical tires, of a brand that's supposed to be sealant-friendly, with dramatically different outcomes. Except perhaps one was manufactured on Friday, just after lunch.
So what do I recommend? First, pick the right tire. I suggest USTs of a brand recommended for use with Stans by your local bike shop. Buy Stans and use it. And instead of just "refreshing" the sealant every few months, replace the tires every 6-12 months. Seriously. It beats walking off the mountain while your competition is racing on without you.
