You need to treat the log siding first. I use a product called boracare sold by nisus corporation It is a borate with a glycol that makes it penetrate into the wood. It must be put on bare wood prior to staining and you need to wait a week or two before staining so it has time to penetrate. weatherall or permachink are good stains and easily accessible. Definitely go to Nisus website and research the boracare. It has a higher concentration than some other treatments sold by perma chink and others, I have used it for years on my log home and I too have log siding on the gables. Just be sure to treat both sides of the siding if possible before putting up or if not possible and it is already hung, just treat the outside very well.