Try getting a power hand planer…We build log homes and use them everyday. It will get it flat enough for you to take a belt or random orbital sander to it to finish it off. Then seal it on both sides with a 2 part epoxy finish. or laminate the bottom side with some sort of cheap countertop laminate. This will keep it from cracking or splitting in two. (Use contact glue on both the log and the laminate, let them dry until they are just barely tacky and slap them together. Roll it down very good with a roller if you have one. The thin laminate can be sanded or filed down around the edges so you should never see it. After that finish the top) Klear Kote is a good brand and very easy finish to use. It can be bought at menards or most hardware stores. Otherwise there are a few places online that sells stuff like that. It can be high gloss or to give it a matte finish, sand it down when it is dry with 800-1000 grit sand paper along with a little water on top. Dry it off and then buff it out with some good car wax. Any questions, let me know.
Troy