Scrape, Sand, or Torch? Removing Wood Finishes

When refinishing wood, it is sometimes necessary to remove the existing finish. Although seemingly a straight-forward process, tightly-bonded finishes or lead-containing paint can cause complications.

Scraping, sanding, wet or dry sandblasting, power washing, and using electrically heated pads, hot air guns and blowtorches are all methods to remove wood finishes—but do you know when to use which technique? Following are a few pointers.

Scraping is effective to remove loosely bonded paint or paint that has already partially peeled from small areas of the structure. If possible, sand weathered surfaces and feather edges of the paint still bonded to the wood. Do not sand if the old paint contains lead.

If paint has partially debonded on large areas of a structure, contractors usually remove the finish by power washing. This method works well for paint that is loosely bonded. If paint is tightly bonded, removal can be difficult without damaging the wood. The pressure needed to debond tightly bound paint from wood can easily cause deep erosion of the wood.

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Old finishes can be removed in a number of ways, but depending on the project, some may be more effective (and safer) than others.

If high pressure is necessary to remove paint, the paint probably does not need to be removed prior to refinishing.

If more aggressive mechanical methods are required, wet sandblasting can remove even tightly-bonded paint. Dry sandblasting is not suitable for removing paint from wood because it severely erodes wood along with the paint and it tends to glaze the surface. As with sanding, power washing and sandblasting are not suitable for paint containing lead.

Some power sanding devices are suitable for removing paint that contains lead; they have attachments for containing the dust. Equipment that has a series of blades similar to a power hand-planer is less likely to “gum up” with paint than equipment that merely sands the surface.

Remember, if the wood has fasteners, planers and sanders cannot be used unless the fasteners are countersunk.

Finally, paint can be softened using electrically heated pads, hot air guns, or blow torches, and then removed by scraping. This process is slow, but causes little damage to the wood. Blowtorches can be extremely hazardous, particularly if the painted wood is on a structure, as the flame can easily ignite flammable materials beneath the exterior surface.

Heated pads, hot air guns, and blowtorches are also not suitable for paint containing lead. Lead fumes are released as a lead-painted surface is heated past 700 degrees Fahrenheit.

For more information, refer to Chapter 16 of the Forest Product Laboratory’s (FPL) Wood Handbook.

The Naked Truth: Use Heartwood to Avoid Heartache

Although untreated, aged, wooden siding conjures up the sleepy seaside resort towns of the Atlantic coast, the chance of your project taking on the silvery aura of a beach-front summer house on Cape Cod is slim. Researchers at the Forest Products Laboratory (FPL) offer the following advice from the FPL’s Centennial Wood Handbook.

Traditional exterior finishes either penetrate wood cell walls or form films on the surface. Penetrating finishes give a more “natural” look to the wood than film-forming finishes—that is, they allow some of the character of the wood to show through the finish. In general, the more natural a finish, the less durable it is.

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An artist’s rendition of the weathering process of round and square timbers. Notice how the interior wood is relatively unchanged over a century.

Although leaving wood to weather to a  natural finish may seem like an inexpensive, low-maintenance alternative to finishing, this approach leads to problems. Wood surfaces erode, some wood species decay, lumber is more prone to split and check, and in most climates in North America, exterior wood develops blotchy mildew growth.

To avoid decay, wood must be all heartwood from a decay-resistant species such as redwood or western redcedar and be vertical grain to decrease the potential for splitting, raised grain, and cupping. Heartwood is the wood at the center of the tree, extending from the pith to the exterior sapwood. The heartwood generally contains gums, resins, and other materials that usually make it more decay resistant than sapwood.

Only limited areas have a climate conductive to achieving a driftwood-gray appearance as wood weathers naturally; the climate along the coast of New England seems conducive to developing the silvery-grey weathered patina that some people desire. Even when the climatic conditions favor the development of a silvery-grey patina, it takes several years to achieve this appearance. Protected areas under eaves will not weather as fast as areas that are not protected, which leads to a different appearance at the top and bottom of a wall.

Do not leave composite wood products, such as plywood, unprotected! The surface veneer of plywood can be completely destroyed within 10 years if not protected from weathering.

Front view of exterior grade plywood siding after 10 years of exposure. The right-hand portion was exposed to the weather, whereas the left-hand side was covered .

Front view of exterior grade plywood siding after 10 years of exposure. The right-hand portion was exposed to the weather, whereas the left-hand side was covered .

Consider instead a penetrating finish such as transparent or clear water-repellant preservative (WRP), a lightly colored WRP, or an an oil-based stain. Natural oils, such as linseed oil and tung oil, can penetrate the microscopic cell walls of the wood and modify the properties of these cells. Cells that are modified with finish typically absorb less water and swell less than unmodified cell walls.

Although the effect will look slightly different than untreated wood, most oils will not leave a film on the surface of the material, unless they are high-solid content semitransparent stains (which may leave a thin film).