Making Sense of Paint Strippers

A Look at Changing Technologies for Going Beneath the Surface

By Barry Chalofsky

Have you ever wished you could magically wipe away the layers of paint hiding the detail in your fireplace mantel or the varnished oak of your door mouldings? Keep wishing. Interior paint removal is always a messy and time-consuming process, particularly in the complex surfaces of an old house. No one method or product works best for all the varied materials and paint layers you're likely to encounter. Fortunately the arcane art of stripping paint has come a long way over the last three decades, with old-house restorers sharing favorite tricks and tools while manufacturers develop better or safer stripping products. In fact, the arsenal of chemicals has grown so much in recent years that it's hard to keep track of all the different technologies. Understanding what the various systems are and how they work, however, will help you choose the ones with the best chances of making your stripping project go as smoothly as possible.

Traditional Methods
All of these general approaches have been around for over a century. Some are low-tech, but each is still widely used for specific kinds of paint stripping.

Mechanical methods, such as hand scraping and power sanding, strip paint by physically breaking the paint bond. These methods are most successful where the wood trim or other substrate is flat, but hand scrapers with curved profiles can be effective for stripping mouldings and window parts. Dry sanding produces the most dust, which is a potential lead hazard (see The Line on Lead, page 61). Therefore, if you opt for sanding, consider using wet/dry sandpaper with occasional water spraying to keep down the dust. Power sanders should be equipped with a vacuum attachment and a HEPA filter.

Heat tools, such as hot-air guns and electric element tools, soften or melt the paint so that it can be scraped off the substrate with a putty knife. Heat tools with an open flame, such as a propane torch, are not recommended because they create a potential fire hazard. While nonflame heat tools operate at lower temperatures, they can still cause fires if used improperly, or scorch the surface, creating a cosmetic problem. Heating and scraping also create paint chips and dust. In addition, if the heat is above 1,100 degrees Fahrenheit, the lead paint may vaporize.

Caustic strippers are pastes or semi-liquids that employ sodium hydroxide (lye), often mixed with trisodium phosphate (TSP), to loosen paint. The alkalis react with the coating, breaking down the organic molecules and loosening the coating from the surface. Caustics eat away the paint layers and, if left in place, may eventually act on the substrate (wood, metal, or other material). They can soften, swell, or decompose wood's cellulose fibers. Generally, surfaces stripped with caustics require neutralizing with a mild acid wash after the stripper has done its job. Caustics can darken some hardwoods, raise grain, or react with metals, such as aluminum. However, they are well-adapted to stripping cementitious surfaces (masonry, plaster) and ironwork. Caustics can burn skin or cause serious eye damage if used improperly.

Solvents
Semi-paste paint stripping products based on solvents have been common since the 1950s. They are fast acting, simple-to-use, and work on a wide variety of paints.

Methylene chloride (MC) or dichloromethethane is one of the most effective organic solvents, but also the most potentially dangerous. MC is fast, efficient, and capable of removing almost any paint. Its small molecular structure makes it able to penetrate the paint layers and break the bond with the substrate. Then, as the solvent attempts to escape, it pushes on the film in a tenting effect that wrinkles the paint up and away from the surface. However, MC also becomes volatile when it interacts with the paint, losing stability and evaporating quickly into the air. Multiple paint layers usually mean applying an MC stripper more than once. (Manufacturers often add wax to the stripper to keep the MC in contact with the surface.) Because of its volatility, MC vapors are harmful to both users and the environment. According to the Consumer Products Safety Commission and the U.S. Environmental Protection Agency, MC is a potential carcinogen. It can also cause dizziness, headache, and lack of coordination.

Other chemical solvents include acetone, toluene, and methanol, often used together. These chemicals are also volatile and can have side affects similar to those of MC. Proper precautions must be taken when using these solvents.

N-methyl pyrrolidone (NMP) has been used primarily in industrial applications for many years, but became popular for paint-stripping products in the 1990s. It works in much the same way as MC--breaking the bond between the paint and the substrate--but its larger molecular structure is less volatile. Therefore it takes longer to remove paint--30 minutes to 24 hours depending on the type of paint and the number of layers. Nor is NMP effective with polyester or baked-on coatings. The good news is that since NMP does not evaporate quickly, it does not need to be reapplied as often as MC. This provides more scheduling flexibility. In addition, NMP strippers don't contain wax, so wax removal is not necessary at the end of the job.


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