I use Lincoln Muddy Buddy Powder as a preventative measure but I’ve also got their cream if I need it. The cream is for broken skin to help sooth and heal.
One of my mares is on a NAF mudfever supplement year round in the belief that the active ingredients bolster the skins resiliance...certainly she has been on clay mud for 3 winters with no problem. (Ive just jinxed myself I know)....
I've had a couple of ponies that have come with mud fever and putting them on a really decent balancer and a one off course of tonic has got rid of it and its always stayed away so it could be well worth a look to see what the supplement contains perhaps
I also had success with the Naf suplement. I bought a cob that had previously failed a vetting due to severe mud fever, causing lameness.
After purchasing him I kept him on heavy clay for several years......I fed the Naf supplement and applied udder cream dialy and he never had a reoccurrence.