Redwood finish won’t hold paint

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Q: The east-facing railing at my home is topped by a 2-inch-by-6-inch piece of redwood, which has always been painted with a dark brown latex paint. One 12-footer out of a total of about 92 feet of railing top will not retain its finish–the paint cracks and peels. The house (and railing) was built in 1963. In the 34 years we have lived here, I have repainted that railing three times. About six years ago, I noted that the surface of this one 12-foot-long 2-by-6 was cracking and peeling. I planed the surface down to bare wood. After priming it and letting it dry thoroughly, I painted this piece with a top-grade latex paint. Two years later, the finish started peeling again. I repeated the procedure: plane to bare wood, prime, dry, paint. This year, the finish is cracked and peeling again. No amateur or professional I have asked has had any notion what could be wrong. The problem did not start until about 25 to 28 years after the house and railing were built. Do you have any idea wh...