Just use a latex caulking. Don't use filler because it will crack as well. Caulking has a bit of flexability so wood can move a little and not crack. Apply a bead to the crack and follow with your finger to get the void filled then wipe with a wet rag. Prime then paint. If you don't prime over the caulking it will come thru as a shiny streak called burning. If you have any other questions you can drop me an e-mail. I don't get to this site that often.