Oxidation: the chalk on your siding that washing can't remove

Every exterior material is slowly breaking down in the sun. Ultraviolet light attacks the pigments and binders at the very surface, oxygen and moisture finish the job, and what is left is a thin layer of degraded material. On vinyl and on painted aluminum siding that layer turns to a fine powder — which is why oxidized siding is also called chalking.

It is gradual, it is uneven, and it is not a defect or a sign of neglect. Every house over a certain age has some. Darker colors show it sooner, and walls that face south or west, where the sun is hardest, go first.

The two problems most houses have are on opposite sides. The shaded north wall stays damp and grows algae and mildew. The sunny south wall dries fast and grows very little — but it takes the UV, so that is where the siding oxidizes.

That split matters when a house gets washed. The north side comes out looking dramatically better, because the growth was the problem. The south side can look no better, or even worse, because the dirt was covering up how faded it had become.

A great many houses in central and northern Wisconsin were sided in aluminum in the 1960s and '70s, and the factory paint on that siding chalks heavily as it ages. Brush against it and your sleeve comes away white.

Old aluminum siding needs a gentle hand. The chalk is the paint, so aggressive washing — and high pressure especially — takes paint off along with it. It is one of the clearest cases on any house for soft washing rather than blasting.

This is the conversation worth having before anyone arrives, not after. Oxidation builds up over years underneath a layer of road film, pollen, algae and mildew. From the street, that layer and the fading blur together into one tired-looking wall.

Take the grime away and what is left is the true condition of the siding. If it has faded unevenly, the unevenness is now visible — a lighter band where the sun hit hardest, a darker patch where a shrub shaded it, the ghost of a shutter that was moved. Heavily chalked siding can also look streaky once part of the chalk rinses away and part stays.

None of that is damage from the wash. It was already there. Washing is what made it visible.

Siding is not the only thing that oxidizes. Colored concrete pavers and stamped concrete fade as the sun breaks down their pigment and whatever sealer they had wears off. Composite decking fades. Plastic trim, fiberglass doors and outdoor furniture all chalk.

The pattern is always the same: cleaning takes off what is sitting on the surface and leaves the color the material actually is today. On pavers that can mean a patio that is clean, but a shade lighter than you remember it.

Vertical black streaks down the face of aluminum gutters — sometimes called tiger stripes — are a close relative. Grime running over the edge bonds into the oxidized paint on the gutter face instead of sitting on top of it.

A normal house wash lightens them but usually does not take them all the way off, because they are partly in the finish rather than on it. If the gutters bother you, say so on the call.

Sometimes, partly, and never by washing. Restoring oxidized siding is a separate process — dedicated oxidation-removal cleaners and hand work, sometimes followed by a protectant — and how well it works depends on how far gone the surface is. Past a certain point the honest answers for aluminum are paint, and for vinyl are living with it or replacing it.

What cleaning does do is slow things down. Algae and mildew hold moisture against the surface, and removing them takes one of the stresses off. A clean wall ages more slowly than a green one.