Yep, I have experience with sarcoids.
The first thing to know is that you need to not just mess around with it. Sarcoids are a pain in the rear because the don't all respond to the same treatment and they can become very aggressive from any irritation, including attempts at treatment. Flies, rubbing from tack etc count as irritation, so be sure that you keep flies and such off of it. Something like war paint or a roll-on fly spray is a good idea so that you can put it right around the sarcoid without putting it on the sarcoid.
Every treatment attempted that fails decreases the likelihood of future treatments succeeding. So, you want to have a vet exam and choose the your best treatment option. One of the commonly used ones that works in lots of instances is Xterra ointment. Your vet can order it f he doesn't already have some. Other options are chryotherapy and surgical removal. The choice really depends on what the sarcoid looks like and where it's located.
Here is a good website to check out: http://pcwww.liv.ac.uk/sarcoid/