This discoloration is most likely caused by a combination of excessive tannins leeching from the wood and mildew growth.
Painting wood shingle siding.
Before you paint condition the siding with a specific type of bonding primer or the final coating will ultimately flake and peel.
If you would like to paint shingle siding consider some critical points.
If you are going to be staining or painting horizontal siding then pine is a good wood to consider as it is easy to finish and will look great.
You ll have to remove the mildew and prime the wood with a stain blocking primer.
Painting cedar however calls for some preparation.
One problem with using pine for your wood siding is that it can be very difficult to find longer lengths of pine that is free of knots.
Wash down the shingles.
Whether it is composed of wood or asphalt your shingle siding is ill suited for adhesion.
The best way to prevent extractive bleed is to select a paint or stain that forms a seal over the cedar.
Some sections of old unpainted shingles and siding may look darker than others.
Powerwash the siding to remove any excess dirt or paint with a low pressure pressure washer.
Preparing wood siding for painting cleaning the siding scraping and sanding loose or peeling paint.
Cedar siding shingles provide beauty for any home but they must be properly coated to ensure a long life.
For old paint in good condition or primed bare wood you ll need about one gallon of paint per each 400 square feet.
Sand off any extreme protrusions from the shingles using a sanding block but do not sand the shingles to.
Divide the area by 400 to get the number of gallons per coat.
Start at the top of the house and work your way down aiming the nozzle downward and keeping it moving constantly.
To determine how much you ll need overall measure the space to be painted and calculate the area in square feet.
You must be careful not to damage the wood while doing so as the water pressure is strong.
Unfinished siding exposed to several weeks of sunlight before painting needs to be sanded.
Sunlight degrades the unfinished wood surface thus it will never hold paint as well as fresh wood.
Spread canvas drop cloths over any shrubbery pavement lawn furniture or decking.
Cedar shingles contain extractives that bleed to the surface when the wood becomes wet and then darken as the wood dries.
If the unfinished wood was exposed more than 3 to 4 weeks lightly sand the surface to remove the thin layer of degraded wood before applying paint.