White foam around tubeless tires

Came across this thread trying to solve my own problem and thought I'd post my solution. I had foam coming out EVERYWHERE around my tire (Conti 5000TL). I had taken it off another wheel, let it sit without cleaning out the (Panaracer) sealant like a numbskull and it hardened. I put new sealant in over the old dried film and the tire just erupted out all the sidewalks when I took it to pressure and cleaned it with water. To solve, I had to remove tire and sealant, and then painstakingly peal off all the old dried sealant (it works better when soaked in water as Panaracer is water soluble). I then remounted tire added more fresh sealant and voila. No more weeping/eruption.

What you are seeing is likely the result of two issues combined:

1. Running tires a very low pressures can damage the cords allowing a path for air and sealant to escape (see picture below).
2. Old sealant can lose its effectiveness meaning the leaks through the broken casing never fully seals.

Damage to the side wall is not ideal, but the tire may still be functional with a sealant refresh. This should be done every few months anyway, depending on the sealant used. If you have never done this, after 10 months you are likely at the far limit of sealant lifetime.

You will need to let the air out, remove the valve core, add sealant, put the valve core back in, then re-inflate. Some people like to remove the tire and clean everything out, which is the best option, but requires you being able to seat a tubeless tire, which can at times be difficult depending on the tire/rim combination and the tools you have available (e.g., external air tank that can be pressurized to deliver a large volume of air to seat the tire).

A thicker latex sealant such as Orange Seal will likely work best to plug the side wall weeping you are experiencing. The tubeless tire pictured below have very weepy sidewalls to begin with, even without cord damage, and the manufacturer recommends Orange as one of the few sealants that will work on this tire.

However, you also don’t want to run too low a pressure with tubeless tires. If the tire flexes excessively, this will break down the casing until it starts to leak (above).

-- The Trouble with ‘Road Tubeless’

This is an issue that often happens when you combine a tyre that doesn't hold air well with a very light sealant. The solution is to add in a sealant that's a bit thicker and chunkier. A mix of 50% NeverFlat and 50% Stan's works very well.

Modern tubeless bicycle tires require a sealant to be added to the inside of the tire. If a small leak develops the sealant seeps out, meets air, solidifies and seals the leak. I believe this arrangement has become popular because earlier tubeless standards like UST required heavier tires.

The white foam dots you see is the tubeless tire sealant leaking out of holes in the sidewalls of the tire, and failing to seal them completely it seems.

what's fascinating is the pattern of holes in the sidewall, I've never seen anything like that before. I think commenters on on your question are correct, there's possibility you have damaged the sidewall by riding with excessively low pressure which has folded the sidewalls at an acute angle.

Refreshed sealant may solve the problem - sealant loses effectiveness over time. Replacing sealant or a tire is not difficult and is relatively inexpensive, depending on the the replacement tire you choose.

Have the bike repair shop check the tire for damage, if the tire is obviously damaged or even suspect I'd opt to replace it as the labor cost for replacing the tire with new sealant will be the same as replacing sealant in the existing tire.

While you are at the bike repair shop, have the rear wheel checked for trueness , you may have suffered some impacts with the tire under-inflated. Also, think about getting yourself a pump with a pressure gauge to avoid future problems if you have been under-inflating the tire.