How to Clean Metal Siding on a Pole Barn or Garage

Why metal buildings streak

Metal pole barns, shops and garages are everywhere outside Louisville. Most are steel panels with a factory paint finish, often a light color on the walls with a darker band near the ground.

That light paint shows everything. Rain runs down the ribs of the panels and leaves dark vertical streaks, the shaded side picks up green, and dust from gravel drives settles on the bottom third.

A metal garage from one of our jobs: the streaked side wall before, and a cleaned end wall of the same building after.

What the panel makers say

The companies that make and paint metal panels agree on the basics.

• Plain water often does it. MBCI, a large metal building panel maker, says washing with fresh water “two or three times per year” keeps a finish in good condition (MBCI panel care guide). Sherwin-Williams, which makes coatings for metal panels, says “simply rinsing the surface with water” handles most buildup (Sherwin-Williams coil coatings).
• For more, a mild detergent. MBCI describes a 5% solution of ordinary “non-bleach” detergent in water, put on with a cloth or soft brush (MBCI panel care guide).
• No wire brushes or abrasives. MBCI says to “avoid wire brushes or abrasives that can damage the coating.” Sherwin-Williams says to avoid “steel brushes or similarly coarse cleaning materials.”
• Rinse it all off. Berridge, another panel maker, warns that “failure to remove all residue from these cleaning steps may damage the film” (Berridge).

The warranty point

MBCI says that “misuse of cleaning agents may void the warranty” (MBCI panel care guide). That is the main reason a metal building is not cleaned like a driveway. Strong chemicals and aggressive scrubbing can cost the owner the paint warranty on a building that is only a few years old.

What cleaning will not do

Years of sun can leave a painted panel faded or chalky, a powdery film that rubs off on your hand. Washing removes the dirt on top. It does not put color back into tired paint, and makers like Berridge point owners to paint companies, not cleaners, for refinishing.

The house next to it

A metal building usually sits next to a house with the same green on the shaded side. On a farm or a rural lot, we can do both in one visit. See house washing and, for vinyl, how to clean vinyl siding.