The problem appears as a splotchy dirt collection. This discoloration can resemble mildew growth, yet will not change color when washed with household bleach.
Stains that come from waxy substances in the reconstituted wood products used to make hardboard siding.
The discoloration can be identified by applying household bleach to rule out possible mildew, tannin, or sulfide staining.
The presence of wax will not be affected by bleach. Apply a few drops of water to the board surface. If the water beads up and runs off the board, a wax condition is indicated and must be removed. Clean the surface with ProClean Professional® PrepWash Concentrate and scrub the entire surface with a scrub brush or high-pressure water wash.
Rinse thoroughly with plain water. Rescrub areas where water continues to bead up on the surface to ensure that wax deposits are removed. Allow surface to dry thoroughly before priming with A-100&Reg Exterior Oil-Based Primer.