Q. We ordered preprimed HardiePlank lap siding to finish a project, but when it was delivered, we discovered that the backs were left unprimed. Normally, we back-prime wood clapboards, but I’m not sure this step is needed with fiber-cement siding, and the manufacturer’s Web site is mute on the subject. Is back-priming necessary?
A. Mark Parlee, a builder specializing in siding installations in Urbandale, Iowa, replies: You’re right to be concerned about the moisture resistance of fiber-cement siding, but carefully following the manufacturer’s installation guidelines will have more of an impact on the siding’s performance than back-priming. According to Rick Crotts, my local James Hardie representative, it’s not necessary to back-prime any of the company’s fiber-cement siding products. He says that when panels are primed in the factory, some primer ends up on the back of the boards as they pass over the rollers, but this isn’t a half-hearted attempt at priming the backs. Hardie actually infuses its fiber-cement substrate with a primer during the manufacturing process to improve paint adhesion, Crotts says.
My company installs a lot of fiber-cement siding (10 to 12 houses per year), and in our experience, paint failure occurs only when the siding wicks up excessive moisture because of poor detailing, such as insufficient clearances at cheek walls and missing kick-out and diverter flashings.