man cannot recreate natural weathering
You will not be able to recreate that weathered and water eroded look easily, if at all. My best advice is to leave it as is because once you start revealing the wood underneath it will not match the distressed look you have. You could trim off the splinters with a sharp knife leaving as much of the original surface as possible, but that's all I might do without actually seeing what you have.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)