I had this experience on my residence about 20 years ago. In order to re-paint, I had to remove peeling paint and prep first. I wound up using a pressure washer, and was able to 'judge' the distance and pressure needed to get the optimum amount of removal without damaging the shakes. (If you do this, I recommend letting it dry out for 4-5 days before applying anything) I also used (at that time) a quality caulk to fill up splits due to shrinkage. Back then, I had to use a good quality latex primer, then two coats of hi-gloss latex paint resulted in another 10+ years. (In hind-sight, it would have been time and materials ahead to replace the shingles, as they were already 25 years old at the time and somewhat dry and on the brittle side). Note: When the time came to do this again, I opted to replace it with a shingle-like heavy gauge dimensional vinyl siding that has performed well and held up to UV, wind, driving rain, and hail storms. The only maintenance is occasional power washing and applying an optional protective silicone spray wax to keep its' luster. It's been up for 13 years now.