I have had similar results using bore butter and the barrel rusting. I don't use it anymore. When I shoot PRBs, spit works the best. I usually shoot a pillow ticking and cut my patches. I will chew on the pillow ticking and get it good and wet. It works better for PRBs than the bore butter does. This of course only when your shooting alot. For hunting on a clean bore, bore butter works good.

For seasoning, are you talking about lapping a new barrell to break it in? For lapping read chapter 6 at http://www.realtree.com/bluelk/

You can probably use JB Bore paste http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=1160 instead of valve lapping compound.