Washing Painted Wood Siding and Trim on an Older House

Why old wood needs the gentlest method

The National Park Service’s guide to exterior paint problems on historic woodwork is direct about pressure: water blasting above 600 psi to remove paint is not recommended because it can force water into the woodwork rather than cleaning the surface, and at worst it drives water through the exterior sheathing and damages interior finishes (NPS Preservation Brief 10).

Its recommended method for cleaning exterior surfaces before repainting is a detergent solution, a medium soft bristle brush and a garden hose rinse, which it calls the gentlest method involving water. It adds that cleaning alone is often enough to put off repainting.

Dirt, mildew or chalk? Each gets a different wash

Brief 10 separates three things that all make old paint look tired:

• Dirt and soot: most loosen with a strong stream from a garden hose. Stubborn spots get half a cup of household detergent in a gallon of water and a medium soft brush, then a thorough rinse.
• Mildew: a fungus that feeds on the paint film and on dirt, thriving where it is damp and shaded, such as window sills, under eaves, around gutters and downspouts, north walls and behind shrubs. The brief’s test: a drop of household bleach turns mildew white right away; dirt stays dirt. Its mildew wash is a cup of non-ammoniated detergent and a quart of household bleach in a gallon of water, scrubbed in, rinsed with a garden hose and left to dry.
• Chalking: old paint breaking down into a white powder, which can streak onto the surfaces below. The brief washes it off with a detergent solution and a medium soft brush.

It also points at the cause of mildew: prune back trees that shade the wall, and fix missing gutters or poor drainage at the base of the house, or the mildew comes back.

Lead paint

Brief 10 notes that many old exterior paint jobs involve one or more layers of lead based paint. Washing is not paint removal, but sanding, scraping and stripping are, and Sherwin-Williams warns that removing old paint by sanding or scraping can create dust or fumes that contain lead (Sherwin-Williams surface preparation).

How we wash wood siding

Wood is on the list of sidings our house wash covers, and our house wash is a soft wash: low pressure and a professional grade sodium hypochlorite (SH) cleaning solution, given time to work, then rinsed. Sodium hypochlorite is the active ingredient in household bleach, the same chemistry Brief 10 uses for mildew, and the low pressure keeps water from being driven into the wood. If the paint is already loose or peeling, the wash will show it, and that is a painter’s job next.

More on older houses: cleaning an older Louisville house.