Poor Donkey!
Where exactly is this sore, Cameron? Is it actually on the donkeys leg, or hoof? Do you think its the result of an injury got in the field maybe?
I have my ponies all injected against for tetanus, because it can be fatal if they are not covered for this, and get a cut infected.
If the sore is clean, I would be inclined to leave it, and not cover it with anything. The problem is getting something to stay on animals legs!! If it did look to have dirt in it, I would gently clean it with warm water.
The vet may give the donkey a jab of antibiotics. The good thing is, there are no flies about to hang about the sore, are there!!!