Cause The process of making hardboard into lap siding makes the board edges much more absorbent to moisture than the faces. Rain, melting snow and condensation run down the face of the siding and collect at the drip or bottom edge. If the edges are not properly sealed, this moisture can be absorbed into the board. The expansion and contraction of the hardboard caused by seasonal temperature changes and moisture absorption create larger spaces in the board which leads to board decomposition and delamination. Other contributors to moisture penetration are areas where nail heads have fractured the board surface and butt ends that were cut and not properly sealed during installation.
Solution Severely damaged boards must be replaced. Some board decomposition can be tolerated; however, these will show as major blemishes on the newly applied topcoat. The use of sandpaper should be done sparingly so as not to break the board surface. Spot priming followed by a full coat of primer is recommended after all traces of surface wax have been removed. (See instructions for "Wax Bleed", Problem #1.)
