Chris,
I'll tell you what I do with pewter buttons. They usually come out of the ground with a lot of chipping and flaking here in the Northeast and you need to be careful because some of them can be pretty valuable. If it is a marked button, to stop the chipping and flaking , I wash the dirt off very gingerly with water and a soft tooth brush (if needed). I let the button dry (you don't want any moisture in when you start coating it) and then coat with clear fingernail polish, first one side of the button, let it dry, then coat the other side. Don't forget the edges too. You might put on a couple of coats. I have also heard that watered down Elmers Glue works well and is easily reversed (removed) by soaking in warm water.
Definitely experiment first using junk buttons. If it is not chipping and flaking, leave it alone.
Here is an example that has had clear fingernail polish for almost 10 years. This is how this button came out of the ground. I knew it had something on it, so I put it in a case as soon as I dug it. Good thing I was careful because I was recently offered quite a bit of money for it.