A. Duffy Hoffman responds: There are a couple of potential problems with the primer that comes on fiber-cement board. First, it's probably been thinned to make it easier to spray, which dilutes both the primer and the mildewcide in it. Also, you don't know how long ago the material was primed. Primer only holds its tooth for 30 to 60 days; after that, the surface should be reprepped.
Unless you have reliable information about when and how the material was primed, I would err on the side of caution. A good substrate ensures a good top coat. If the substrate fails, so will the top coat. Even if you could get the siding manufacturer to cover the cost of the paint, it wouldn't cover your labor cost to scrape, sand, and recoat....
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