Primers and Paints for Wrought Iron, Cast Iron and Steel

This document includes general information on primers and paints to be used on interior and exterior wrought iron, cast iron and steel surfaces.

Note: The selection of a suitable primer and paint for metals depends on many factors including the type of metal to be coated, the type of surface preparation to be used, environmental and surface conditions, desired appearance and performance requirements, method of application, and type and level of exposure. Consult a paint manufacturer along with the regional historic preservation officer before making the final selection.

The primary purpose of paint is to protect the metal from deterioration. To do so, paint manufacturers have developed paint systems which are made to work together to protect the metal substrate. These systems include primers and appropriate, compatible top coats which can vary depending on the substrate, environmental conditions, and can vary between manufacturers. As a result appropriate primers and compatible top coats, both from the same manufacturer should be used.

For information on paint removal from metal, surface preparation and application procedures, see the following:

For guidance on paint removal from iron and steel, see 05010-05-R “Cleaning/Removing Paint from Wrought Iron, Cast Iron and Steel Using Mechanical/Abrasive Methods, “Removing Paint from Wrought Iron, Cast Iron and Steel Using Thermal Methods” and “Removing Paint from Wrought Iron, Cast Iron and Steel Using Chemical Methods”.

For additional information on the history, properties and uses of paint, see 09900-01-S. See 09900-07-S for general guidelines on painting interior and exterior surfaces.

Primers for wrought iron, cast iron, and steel

Characteristics:

• Primary function is adhesion.
• Must bond well to substrate and intermediate coat.
• Should have enough chemical and weather resistance to protect the substrate before application of next coat.
• Should be compatible with intermediate and top coats.
• Should be compatible with paint/rust removal methods.

Types

Oil/Alkyd primers:

• Advantages:
  • Bond well to most surfaces even if surface preparation is substandard.
  • Compatible with oil finish coats.
• Limitations:
  • NOT compatible with finish coats of vinyl, epoxy or other synthetic polymer. Solvents in these systems attack and soften these primers.
  • Limited corrosion resistance.

Mixed resin primers:

• Advantages:
  • Bond well to most surfaces, though adequate surface preparation is important.
  • Compatible with most finish coats.
  • Good alkali resistance.
  • Some corrosion resistance.
• Limitations:
  • Compatible with a specific range of top coats.

Resin same as topcoats:

• Advantages:
  • Effective when surface is properly prepared.
  • Some corrosion resistance.

Inorganic zinc:

• Advantages:
  • Outstanding bonding characteristics when surface is cleaned and roughened. Compatible with most finish coats.
  • Effectively resists disbonding.
  • Excellent resistance to underfilm corrosion.
  • Effective in protecting the metal without the help of a finish coat.
• Limitations:
  • NOT acid or alkali-resistant.

Wash primers (Suitable for use on steel, aluminum, zinc, cadmium, chromium, tinplate, and terneplate):

• Advantages:
  • Provides a smooth, durable, uniform base for finish coat application.
  • Compatible with the following top coats: vinyls, phenolics, alkyds, nitrocellulose and oil-type products.
• Limitations:
  • NOT recommended for surfaces subject to temperatures above 150 degrees F (66 degrees C).

Conversion coatings (Oxidizing solution used on zinc, iron, aluminum and magnesium [formulation will vary depending on type of metal]. This type of coating is usually factory-applied.):

• Advantages:
  • Excellent corrosion resistance.
  • Good adhesion to subsequent paint coats.
• Limitations:
  • Subject to deterioration if overheated.

Zinc chromate (Used on aluminum, magnesium and ferrous metals):

• Advantages:
  • Good corrosion resistance.
• Limitations:
  • Not suited to highly acidic environments.

Zinc-rich coatings:

Advantages:

• Good corrosion resistance.

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Paints for wrought iron, cast iron, and steel

Characteristics:

• Should be compatible with primer.
• Intermediate coat should uniformly bond the primer with the top coat.
• Intermediate coat should have enough chemical and weather resistance to protect the primer and substrate.

Types

Oil-based/Alkyd enamel:

• Advantages:
  • For normal to severe weather conditions; provides good abrasion and dirt resistance.
  • Suitable for both exterior and interior uses.
  • Good bonding characteristics.
• Limitations:
  • Alkyds are not good in a continuously damp or chemically corrosive environment, nor are they solvent resistant.
  • Limited alkali resistance.

Baked phenolic:

• Advantages:
  • Excellent resistance to acidic environments.
  • Excellent resistance to water.
  • Excellent resistance to strong solvents.
  • Low material cost.
• Limitations:
  • Low alkali resistance.
  • High labor cost for application.

Epoxies:

• Advantages:
  • Good adhesion.
  • Good chemical resistance.
  • Good abrasion resistance.
  • Good alkali resistance.
• Limitations:
  • Sensitive to chalking under exterior exposure.
  • Sensitive to color fading.
  • Weak in acid.

Acrylics (thermoplastic and thermosetting coatings):

• Advantages:
  • Moderate cost
  • Good resistance to degradation from ultraviolet light.
  • Suitable for both interior and exterior use

Vinyls (used primarily as intermediate coats):

• Advantages:
  • Good alkali and acid resistance.
  • Excellent water resistance.
  • Low chalking rate.
• Limitations:
  • Limited solvent and heat resistance.
  • Inferior to alkyd and epoxy coatings - lower adhesive strength.
  • Sensitive to intercoat contamination.

Inorganic zinc:

• Advantages:
  • Excellent weather and solvent resistance.
  • Excellent resistance to underfilm corrosion.
  • Resistant to petroleum products.
• Limitations:
  • Limited chemical resistance.
  • Not suitable for strong acid or strong alkali environments.

Organic zinc:

• Advantages:
  • Protects against corrosion.

Furan:

• Advantages:
  • One of the most versatile and resistant of organic films.
• Limitations:
  • Poor adhesion to steel and any primed surface.
  • The film gets very hard after curing, making it extremely difficult to maintain them.

Urethanes:

• Advantages:
  • Excellent gloss and color retention.
  • Preferable to epoxy protective coatings or primers.
  • Available in a wide variety of formulations for different surface types and conditions.
• Limitations:
  • Comparable to epoxies and vinyls in resistance to corrosion.
  • Some tend to yellow when exposed to sunlight.
  • Expensive.

Silicones:

• Advantages:
  • Excellent heat resistance.
  • Excellent color and gloss retention.
  • Available in pure or modified form (a mixture of two coating types)
• Limitations:
  • Expensive

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References:

• Margot Gayle, David Look, John Waite. Metals in America’s Historic Buildings. Washington, DC: National Park Service, 1995.
• L. William Zahner. Architectural Metals. New York: John Wiley & Sons, 1995.