Before kubota changed their policy they were fixing them. I had several complaints of the same issue with the same series tractor (Grand L 10, 30, and 40 series). Now, the policy is changed (or was one year ago), such that if it was out of warranty, it was out of warranty and not much could be done.
It looked like when they prepared the metal for paint, the prep wasn't good in the seam area, and the paint would fall off and/or rust.
The ideal way to fix it is to replace the fender(s). Or pull them off and have a body shop repair them if they aren't too rusty. I sent a couple to a local body shop who took good care of them.
On your L4701 if its less than 24 months after sale date, take it to the dealer and ask them to file for fender replacement. Once you hit 24 months it's on you at that point and I don't recall fenders being cheap.