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Preserving Exterior Metal Elements on Your Historic Building | Wisconsin Historical Society

Guide or Instruction

Preserving Exterior Metal Elements on Your Historic Building

Preserving Exterior Metal Elements on Your Historic Building | Wisconsin Historical Society

Your historic house or building could have exterior metal surfaces and features such as a metal roof, metal flashings, cornices, decorative window hoods or wrought iron. These metal surfaces require repair and rehabilitation from time to time. Preserving, maintaining and repairing these architectural metal elements will be an important part of your efforts to retain the historic look and architectural integrity of your house or building.

Evaluate the Condition of Your Metal Elements

EnlargeBay window

Milwaukee County. The roof on this bay window is copper and thus does not require painting. When copper is exposed to the weather it develops a patina which is green in color. Source: WHS - State Historic Preservation Office.

As part of your ongoing house or building maintenance efforts, you should carefully inspect your architectural metals. Note any damage to the metal parts, and evaluate the condition of the paint, adjacent caulk and surrounding mortar. Most of the metal elements on your historic structure are either tin or terne metal.

Terne metal is a historic metal that was used extensively on historic homes and commercial buildings. Terne coatings came into use in the late 1800s. Terne metal was coated with a corrosion-resistant mixture of around 15% tin and 85% lead. Terne and tin metals must always be painted, so the condition of the paint on your metal elements is especially important.

Some of your metal elements may be copper, which you should never paint. When copper ages, it develops a dark greenish patina from dirt and oxidation that actually protects the copper metal. Your bronze and brass elements will also develop a patina as they age that is a darker shade of their original color. You should not remove this patina because it adds value and integrity to your metal elements and protects the primary metal underneath.

Clean Your Metal Elements

Thin metals can deteriorate, so it is crucial that you clean these unique features carefully. Use this process to clean bare metal that you will be painting:

  1. Use a liquid biodegradable dishwashing detergent and hot water to clean the metal.
  2. Lightly scrub dirt from the metal with a nylon-bristled scrub brush.
  3. Use a toothbrush to scrub dirt from crevices and corners.
  4. Rinse the metal with clear water.
  5. Towel-dry the metal thoroughly.
  6. Use #0000 steel wool to lightly abrade the metal surface. The steel wool will remove the patina and additional dirt so the paint will adhere to the surface.

If the paint on your metal elements is failing, you should use a liquid stripper to remove the paint. Avoid using paint strippers that contain methylene chloride. These strippers are highly toxic and can actually create pits in historic metals. Instead, use an environmentally sound and safe soy- or citrus-based stripper to remove the paint from your metal elements.

Remove Rust from Your Metal Elements

Cleaning rusted exterior metals is much like auto body restoration. You can use chemicals or abrasives to remove rust on these elements. The most abrasive methods for removing old paint, rust and dirt are wire wheels, electric sanders and walnut shell blasting. These methods are appropriate to use on heavy wrought iron elements, like roof cresting and fences. However, these abrasive methods can ruin your delicate tin, terne metal and copper elements.

TIP: Never sandblast historic metals. Sandblasting heats and distorts the metal and can permanently ruin the metal. You may be able to use soda blasting or walnut shell blasting on pressed tin ceilings and other metals that do not have very thin walls.

To remove rust from iron and steel, you may want to use a rust converter. A rust converter is a water-based primer that contains an organic polymer and tannic acid. The organic polymer provides a protective primer layer. The tannic acid chemically converts rust to a stable, black, protective polymeric coating that serves as an excellent primer for oil, urethane and epoxy-based paints.

Paint Your Metal Elements

EnlargeBay window

Racine County. The metal roof on this bay window has peeling paint. In order to maintain the roof's integrity, the loose paint should be scraped and new paint reapplied. Source: WHS - State Historic Preservation Office.

When you are preparing historic architectural metals for paint, the key thing to remember is that paint will not stick to dirt or rust. Follow the instructions above to clean and remove rust from your metals before you apply paint to them. If you have metal elements that are attached to wood or masonry surfaces, you will have to caulk these areas before you apply paint. Be sure to remove any old or failed caulking, and use a new caulk that is compatible with your paint.

For all of your metals other than copper, brass and bronze, you must use an oil-, urethane-, or epoxy-based paint that is designed for use on metal. These paints are often called "direct to metal" or DTM.

Follow these steps to paint your historic metals:

  1. Strip off any old paint and thoroughly clean the metal according to the cleaning instructions described above.
  2. Allow the metal to dry completely.
  3. Wipe the metal surface with paint thinner and allow the paint thinner to evaporate.
  4. Apply an at least three coats of DTM paint using a natural-bristled brush.

TIP: If you are stripping and painting a large area, it is best to do these tasks in small, workable sections that can be completed in a short period of time. When you strip paint from a metal surface, it will begin to oxidize in as little as a day. The metal will not hold the paint well if oxidation has already begun. Therefore, you should only strip an area that you will be able to repaint within a day’s time.

Repair Your Metal Elements

You might be able to make some repairs to your metal elements yourself, while other repairs will require the expertise of a professional.

You can use an auto body filler to fill dents and some holes in your metals. Be sure to remove all the paint and dirt before you apply an auto body filler, and finish the job by sanding the filled profile. Auto body fillers are only as good as the paint that protects them, so you must maintain the paint if your filler is to last. If you do not maintain the paint, your repairs will only last 5 to 10 years. This type of repair can help to stabilize the metal for future restoration.

If areas of your architectural metal surface are gone, you should solder or weld like metal onto the missing surface area. This is a highly skilled task, so you should hire a professional who has years of experience repairing historic metals. You or the professional you hire may have to carefully remove the metal element so it can be worked on in a workshop environment.

If you have terne metal or tin window hoods, bracket work or other details, they are probably attached to your structure with metal flanges or straps. These attachments can fail over time, so you might need to reattach them to your metal element by soldering or welding.

Cast-iron elements like window hoods are generally found on masonry buildings. They are hung on cast brackets attached to the structure. If you have a cast-iron element, you may need to take it off the brackets for repairs. Often these brackets are attached to a wood sleeper — a wood frame that you cannot see because it is embedded in the masonry. These wood sleepers can rot or get loose. If you have a failed wood sleeper, you should replace it with a new sleeper made of white oak. The new sleeper must be big enough to be friction-fit into the sleeper crevices by lightly tapping it with a hammer. You can then slip your repaired and pre-painted hood over the bracket and secure it at the bottom. 

Replace Your Damaged Metal Elements

If your house or building has missing or unrepairable architectural metal elements, you should first try to determine if the element is still being made. Most architectural metal elements were made so long ago that they are no longer available today. However, a few of the original manufacturers of these elements are still in business today, so you might be able to find an exact match for your metal element.

Many reproduction architectural metals will not match and are not made as well as the historic metals on your house or building. Having architectural metal elements custom made, when they cannot be repaired, is the best option but can be very expensive. Because of the cost of replacement, maintaining and repairing architectural metals is all the more important.

The information presented here is not intended to provide comprehensive technical advice or instructions on solving historic preservation issues. Any information contained or referenced is meant to provide a basic understanding of historic preservation practices. Read full disclaimer.