Stains that come from waxy substances in the reconstituted wood products used to make hardboard siding. When the substrate is painted, these staining substances bleed through the paint; they can even bleed through some ordinary primers, possibly causing dirt pick-up, mildew and/or poor paint adhesion (see Dirt Pick-up and Mildew).
Failure to apply a proper primer to hardboard before applying the top coat. Allowing hardboard siding to weather before being painted.
To treat or prevent, apply a quality exterior acrylic latex primer; follow with a coat of high quality exterior acrylic latex paint. Two coats of top quality acrylic exterior house paint are recommended for best results. Some hardboard grades have an adequate factory primer and need only a quality paint applied. Low quality, highly pigmented flat paints are more prone to wax bleed than are higher quality paints.