Peeling on galvanized metal
 
Cause

As a rule, paints do not adhere well to smooth metal surfaces. Galvanized metal, a product used to make corrugated sheets, gutters and downspouts, is not only smooth, but the metal is coated with a thin to heavy layer of oil to prevent white rust.

When oily galvanized steel is painted during cold and highly humid conditions, the curing of the paint film is retarded, resulting in wrinkling and loss of adhesion (see insert).

Alkyd paints that are applied to galvanized metal will produce an acceptable paint film initially, but will quickly fail. The zinc of the galvanize will chemically react with the alkyd binder of the paint, which leads to extreme paint peeling.

When galvanized metal is allowed to remain exposed to the atmosphere, the protective zinc metal coating will oxidize into a white rust until eventually the zinc metal is depleted. The underlying steel will then rust to the offensive red color.
 
Solution

The protective layer of oil must be completely removed. Solvent wiping using a water soluble cleaning agent or VM&P naphtha is recommended for small manageable areas. Many solvents, including mineral spirits, leave a thin layer of residue which leads to adhesion failure of the paint film.

Weathered galvanize metal develops a layer of "white rust" which must be removed to ensure good adhesion of the applied paint. Wire brushing is recommended.

Iron rust must be removed by wire brushing followed by sanding with a productive grit sandpaper. Severely rusted gutters should be replaced.

CAUTION: When removing "white rust," wear a face mask to avoid inhalation of dust particles.
 

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