Karen, I, normally, do as Steve suggested. Dry the boards out, outside and then beat on them to get the bugs to leave. I, usually, use a wirebrush to get dirt and grime off of the boards. If I don't like the color to which the boards have weathered, then I'll use a belt sander to lighten them up. Then I just finish with a coat or two of clear polyurethene. Sure hope that's spelt correctly.
If the board is rotten and structurally unstable, I don't know of anything that will return the strenth to it. But for a fireplace mantel you shouldn't need much strenth and as Steve said, all the damage just adds character.
Coating old barnwood with anything will change the color of the wood, so you might want to coat a scrap piece first to see it that's the color you want the end product to look like.