The Eventing Vet mentioned cleaning stud holes with WD40 but I thought I would just mention another way to make the job easier.
I discovered that stud holes are easier to clean when you have washed the horse. I now wash the horse and while its feet are still wet, I will do the stud holes. The are really easy to do when wet and then plug with cotton wool or cigarette filters overnight.
WARNING NON BHS METHOD! – I have been known on a particularly easy going horse to pick its feet up and direct the nozzle of the hose into the stud hole to clean it out which also works well!