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.
Because fiber cement is a hazardous material to
sand, to me it makes more sense to prep the
substrate correctly in the first place. Here's what
I would do.
First, wash the siding with Pittsburgh Paint's
MildewCheck. This is better than using a bleach
solution, which dissipates within about 48 hours,
allowing mildew spores to once again begin growing.
MildewCheck leaves a longer-lasting film of
mildewcide on the surface. Next, I would lightly
etch the surface with 150- to 320-grit sandpaper,
then prime with a good acrylic primer, followed
with a 100% acrylic top coat.
Duffy Hoffman is a painting contractor and
restoration consultant in Pipersville, Pa.