I have a horse who has lived in wedge pads for 6 or 7 years and lives (read: wallows) in mud pretty much year round given our location (PNW).
We have to be careful with what we put in the pads, as my guy canāt tolerate anything that āsolidifiesā or hardens at all. Silicone and the like absolutely cripple him midway through his shoeing cycleā¦presumably because his feet flatten and kind of warp as they grow. Even the dental impression material is a no-go. So weāve had the most luck with oakum. This allows him to have a packing material that stays in place and keeps rocks and mud out without creating pressure points.
To that point, though, I would think that most packing materials will eventually leave enough space to allow rocks and mud in eventually as the foot grows, assuming a normal-growth foot. Weāve done what essentially amounts to silicone beaded along the heel edge of the shoe to seal the pad a few times over the years, but Iāve never found that the seal it creates lasts long term.
But with all of that said, I havenāt had problems because of mud getting in under the pads, knock on wood!