COVID-19 Updates: For the most up-to-date information on accessing our services learn more here.

Preparing Metal Surfaces for Paint on Your Historic Building | Wisconsin Historical Society

General Information

Preparing Metal Surfaces for Paint on Your Historic Building

Preparing Metal Surfaces for Paint on Your Historic Building | Wisconsin Historical Society

Your historic house or building might have exterior metal surfaces that need a new coat of paint. Some common metal architectural elements found on historic structures include tin cornices, decorative tin window hoods, terne metal roofs, roof flashings and wrought iron. Terne metal is a historic metal used extensively in historic houses and buildings. It is made with tin and lead and must always be painted.

You should use an oil- or epoxy-based paint that is designed for metal to paint all of your metal surfaces.

EnlargeRemove rust

One way to remove rust is to use a wire wheel. Here a wire wheel is used to remove rust from a cast metal sill. Source: Bob Yapp

Repair Your Metal Elements

Before you attempt to remove rust or paint from your metal elements, assess the condition of the metal surfaces. Thin metals can deteriorate, so it is crucial that you take extra care to prepare these unique features for paint. Look for dents, holes and other necessary repairs.

You can repair dents and some holes in your metal elements by filling them with an auto body filler. Use sandpaper to sand the filled profile. For some repairs, you might need to weld or solder new metal to the old metal.

Prepare the Surface for Paint

EnlargePrimer coat

After wiping down the metal surface with mineral spirits, use a metal oil paint as the primer coat as seen here on a metal sill of a historic greenhouse. Source: Bob Yapp

When you are preparing metal surfaces for paint, the most important thing to remember is that paint will not stick to rust or dirt. You can remove old paint, rust and dirt with chemical or abrasive methods. For delicate metals, chemical paint and rust removers are your best option. The more abrasive methods include wire wheels, electric sanders and walnut shell blasting. These abrasive methods could ruin your delicate metal elements, but they are appropriate methods for heavy wrought iron and thicker-gauge metals.

For rusted iron and steel, consider using a rust converter. A rust converter is a water-based primer that contains tannic acid and an organic polymer. The tannic acid reacts with the rust and chemically converts it to a stable, dark-colored protective polymeric coating. This coating serves as an excellent primer for both oil- and epoxy-based paints.

Just before you apply paint to a metal surface, wipe the metal surface with mineral spirits. The mineral spirits remove minerals and debris from the surface and improve paint adhesion.

Prevent Oxidation of the Metal Surface

When you are cleaning a metal element to prepare it for paint, clean only small, manageable areas of the surface at a time. The freshly cleaned metal will begin to oxidize in as little as 24 to 48 hours, and oxidized metal will not hold paint. Therefore, each area you clean should be small enough so you can finish the surface within one or two days. Never allow freshly cleaned metal to remain unfinished for more than two days.

If you need to prepare large areas of metal for paint, use this two-step approach:

  1. Strip all the paint, rust, and dirt from an area equal to one day’s work.
  2. Prime the cleaned area within 24 hours of stripping it.