Painting Steel Buildings: An Overview

Published June 15, 2016 by Whirlwind Team

painting steel buildings

While metal building panels can be coated in any color at the manufacturer, you can paint steel panels on-site as well. The owner may need to change the color for one reason or another, the paint may need refreshing, or damage needs to be repaired.

There are quite a few benefits to painting a steel building. It will improve the durability of the steel to have an extra protective coating. It’s also a good way to go green.

A light-colored roof can help reduce the heat island effect and lower energy costs by reflecting solar rays away from the building. If you are applying for a LEED designation, energy efficient coloring will help you gain the points needed. Painting can also prolong the life of an older roof.

New paint can make the building stand out (or blend in). Plus, it’s just so purty when it’s done!

When to repaint

It’s time to repaint a metal surface when:

  • It’s peeling or flaking
  • Paint is lifting off the surface
  • There are patches of rust or metal corrosion
  • Existing paint looks chalky
  • Color is fading

Over time, weathering due to severe storms or intense sunlight can cause paint to fail. Metal buildings don't need a lot of maintenance, but this is one area that needs attention every so often.

Rust removal tools

Always wear gloves and protective eyewear when performing this type of activity.

Rusted areas will need sanding and scraping. Some of these tools are good for removing loose and flaking paint as well.

Sandpaper and wire brushes are the basic tools of the job. Use 80-grit sandpaper to remove as much of the rust as possible. Follow up with 120-grit sandpaper for surface smoothing.  If you need to repaint a glossy, slick, or non-porous surface, 120-grit sandpaper is also useful to roughen the surface and improve the adherence of the paint.

A wire brush is helpful for scraping off the roughest of the paint and rust. You can use a basic brush or a wire brush wheel attached to a drill. You may want to consider sand-blasting with fine sand or other abrasive material and a high-pressure air compressor.

Alternatives to scraping and sanding include chemical treatments, a good choice for large areas. Organic rust converters are gel-like organic compounds that dissolve rust and act as a rust preventative. Be careful with the application because it’s easy to create a surface with visible brush strokes.

Other rust removal chemicals can be harsh. They often contain phosphoric or hydrochloric acid. Use a respirator. After rust removal, carefully rinse, clean, and dry the metal before painting.

Types of paint

Painting metal protects it from rust; some paints and primers provide extra protection and rust-proofing. You can even get coatings that are specifically for roof panels or wall panels. Some are developed to reduce volatile fumes and meet sustainability standards.

Water-based paint

One of the newer paint formulations for metal is water-based paint, something that has been available for other surfaces for years. It is easy to clean up, and the vapor transmission rate is lower.

Water-based paint forces moisture up to the surface and replaces it with rust-proof chemicals to seal the surface. You may see a slight watery sheen on the top of the paint. Manufacturers advise applying at 50 degrees F and rising with no rain in the near term forecast.

Water-based paint is not recommended for metal that will be immersed in water, acids, alkalis or solvents. It is also not intended for Kynar® or other PVDF (polyvinylidene fluoride) resin coatings.

Polyester resins

This is the least expensive of the coatings on the market for metal. It is commonly used for exposed fastener panels and gives the surface a medium to high gloss. It tends to fade after five to seven years of direct sunlight, which is more noticeable in dark colors.  Warranties are for three to five years and rarely cover fading or chalking.

Silicone-modified polyester

Also known as SMPs, these are polyester resins blended with silicone additives for improved performance. Higher silicone content equals more durability. It’s available in medium and high gloss colors that resist fading and chalking better than standard polyester paint. It is often warranted against excessive fading and chalking for 10 to 20 years.

PVDF (polyvinylidene fluoride) resins

PVDFs are the most technically advanced (and expensive) finishes. It is sold under tradenames such as Kynar 500® and Hylar 5000®.

PVDFs provide a smooth and dense medium gloss finish with excellent durability. It has long lasting resistance to fading and chalking, even in intense sunlight. It also resists dirt retention. PVDFs are great for coastal environments where it holds up better than other paints. Warranties go for 20 years or more with additional warranties of 10 to 20 years against excessive fading.

Reflective finishes

If the owner wants to reduce air conditioning costs, you can apply paint with a reflective finish. White and light colors with reflective finishes can reduce cooling loads by up to 30%. Even dark colors can be made more cost-effective with special formulations that selectively reflect infra-red (IR) and ultraviolet (UV) rays.

Granular coatings

Crushed stone or ceramic granules are blended into an acrylic resin and applied over a special primer. You seal it with acrylic sealant. Multicolored granules can give the appearance of asphalt shingles. Granular coatings can protect against scratching from foot traffic as well as denting from hail. Any small dents tend to be concealed by the rougher texture.

Steps for repainting or recoating

First up is surface preparation.

You need to remove rust and other loose components mentioned above. Any holes or cracks can be repaired with auto-body filler before sanding. If you use an organic rust converter, you don't need to sand or wire brush before application.

Next, clean the entire panel with mild soapy water or use a pressure washer to remove dirt, debris, chalk, and rust dust. If you need to de-grease, use a stronger detergent and thoroughly rinse. You can remove light "flash rust" with a cloth dampened with paint thinner or a commercial surface preparation. Mold and mildew should be washed away using a solution of three parts water and one part bleach. Allow the surface to dry completely and apply a sealant.

As you paint, work from the top to the bottom. Chalky or oxidized surfaces may need priming. If so, use a solvent-thinned primer developed for metal. Spot priming is acceptable for small areas of weathering or rust repair.

If you prime, allow it to dry completely before applying the paint. An acrylic-latex top coat can help toughen against weather and provide a semi-gloss finish.

Safety at height

Painting a metal roof has a few cautions to consider.

  • Don’t walk on a wet roof. Obviously, it will be slippery.
  • Step on panels only where they are supported by an underlying structural member.
  • Avoid walking on the flashing or in the gutters.
  • Careful walking along the edge.

Paint steel buildings as needed to refresh the look or to replace fading or flaking paint. If rust is visible, definitely repair and paint. Done correctly, it will look as good as the coating from the manufacturer.

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