I don't know where people get that one but all it will do is give you a stinky horse.
You need to make sure the horse is in a clean dry pen/stall.
First you need to clip the area free of long hair (as short as possible is best). The treatment that we use out here is a mixture of Desitin (yes, baby butt cream), nitrofurizone ointment, and Dexamethasone. This is applied once daily to the affected areas. Each day wipe the old off with a paper towel (don't wash, just wipe) and apply a new coat. Don't wrap the leg leave it open to the air, thats where a clean dry pen comes in. Do this daily for about a week or until the sores are healed. Most time your local vet clinic will have "Scratches" medication made up or will make it up for you and it normally is pretty cheap to get. Caught early it clears up pretty quickly, the most important thing is not to wrap no matter what treatment you use. the area needs air. If this doesn't work there is also oral antibiotics that can be turned into a paste and put on in some severe cases. We had to do this once with a horse that was prone to getting it along with oral meds.
Pastern Dermatitis (scratches) is really not something to mess around with, left untreated properly it can get severe quickly and become really painful, making the leg swell and the horse becoming lame.
Hope this helps