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.

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.

Weathers galvanize develops a layer of "white rust" which must be removed to ensure a 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.

Recommended Sherwin-Williams Coating System

-Galvanized (new)

Two coats Exterior Latex House and Trim Paint

-Galvanized (weathered)

Two coats Exterior Latex House and Trim Paint

-Galvanized (red rust)

One coat DTM Primer/Finish

Two coats Exterior Latex House and Trim Paint

Cause

The appearance of mildew depends on various conditions, which are:

Continuously high humidity. As the humidity increases, mildew growth becomes more rapid.

High average temperature. Optimum temperature for mildew growth is 77F.

Poor ventilation. Still air increase mildew growth.

Composition of surface. Mildew will grow on any surface that provides a nutrient, even dirt.

Light colors of paint film. Colors that do not absorb the sun's heat provide a surface for mildew growth. Dark colors become hot and discourage mildew growth.

Solution

Wash mildewed areas with a solution of one part household bleach and three parts water. This will destroy mildew and bleach stains caused by mildew growth. Apply solution by brush or a garden sprayer apparatus. Heavy mildew may require additional applications, and scrubbing may be required. Flush area with clean water to remove bleach solution. Allow to dry thoroughly,

Recommended Sherwin-Williams Coating System

Spot prime bare areas

Cause

Peeling is caused by poor surface preparation, and refers to the removal of strips or sheets of paint due to loss of adhesion. The loss of adhesion could be the result of: painting over a coating containing wax; an excess buildup of moisture within the wood; painting over an old, smooth coating; or painting over heavy chalk surfaces.

Other surface conditions or contaminants known to affect the adhesion of newly applied topcoats are oil, grease, various forms of pollution such as dust, dirt, and mildew, applying paint over hard glossy surfaces and excessively thick film topcoats.

Peeling may also be the result of applying latex paint below its recommended application temperature.

Solution

Proper surface preparation will prevent peeling of the topcoat. If a peeling situation persists, the loose paint must be completely removed. Prepare surface by removing or abrading the problem surface. Sand all bare wood to fresh wood.

Remove surface contaminants with naptha solvent, detergents, and water. Glossy surfaces must be scuff sanded. All surfaces must be rinsed with clean water and allowed to dry thoroughly, Exposed wood areas must be sanded to fresh wood and primed with an alkyd or latex primer.

Recommended Sherwin-Williams Coating System

One coat A-100 Exterior Wood Primer

Two coats Exterior Latex House and Trim Paint

Cause

Red-colored woods contain a water soluble dye called tannin. Tannin is not soluble in most solvents. Application of latex topcoats directly to these red-colored woods may develop a red-colored stain on the finish coat. If the dry film is intact and discoloration occurs at a later date, then staining is being caused by water moisture within the board wall. This moisture will migrate and eventually carry staining substances from within the wood to the surface of the paint film.

Some cedar woods contain colored dyes such as tannin which is water soluble, and other color extracts which are soluble in mineral spirits. In these instances, a primer that has discolored should be spot tested with a second coat of primer or a latex topcoat to determine which will successfully block future discoloration.

Solution

If dry film is intact and discoloration is occurring at a later date, then internal moisture is the culprit. The moisture source must be located and corrected. Remove stains by washing with a solution of equal parts alcohol and water. Allow to dry thoroughly.

New red-colored woods must be sealed with an undercoater that will function as a barrier coat, preventing staining of the topcoat. This undercoater can be either a solvent or latex primer, depending on severity of available tannin dyes in the wood.

Although the latex primer contains water, the product chemistry is designed to minimize surface discoloration by tannin bleed. Extreme cases of tannin bleed may require one or two coats of alkyd primer to prevent discoloration.

Recommended Sherwin-Williams Coating System

Light-colored woods

One coat A-100 Latex or Alkyd Exterior Wood Primer

Two coats Exterior Latex House and Trim Paint (Flat, Satin, or Gloss).