This is a tongue & groove detail using Windsor One T&G product. Any seems would be covered in the paint prep with bondo. Different manufacturers offer different lengths but sometimes you can get away with full runs soffit to soffit to avoid seems. With T&G there is a high paint maintenance as the wood settles, lacquer paint will often pull or crack and needs routine touch up. Something to keep in mind. You can always use a flat stock (MDF) and route the grooves in to avoid separation. Good luck!
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BM Blue Note 2129-30
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