Although felt soles, because of their material, (very few, are "real felt" anymore), are of course a great place for Dymethalgradue to hide and be transported from one body of water to the next.
But, while "basting your boots in salt water", don't forget that your waders, (cuffs, gators, neoprene booties, etc.), as well as your net fabric, the cuffs on your shirt/coat (if you got either wet, perhaps landing a fish), even something as simple and small as a braided loop connector................. can ALL carry unwanted debris from one stream/lake to the next.
It's not only the felt soles, that are transporting these water problems. The "crud" can hide and adhere to almost any surface so while salt dipping, or bleaching, your felt soles also don't forget to give the same treatment to the "last" on your boots, the tongues, the laces and if you have "vents" on your boots as most of us do, also include the INsoles as well.