Flexner on Finishing: Refinishing Furniture

Stripper types. There are five categories of strippers. From left to right: high-percentage methylene chloride, low-percentage methylene chloride, strong solvent and no methylene chloride, refinisher (no methylene chloride and no wax), and NMP (n-methyl pyrrolidone).

Repair, strip and refinish to restore old pieces.

By Bob Flexner
Pages: 58-60

From the August 2011 issue #191
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Refinishing is a topic worthy of an entire book. In fact, restoring furniture includes all of woodworking and finishing because all skills may be called into play. In lieu of a book, here are some not-so-random thoughts.

1 ■ Refinishing is a good thing, not bad as suggested by the “Antiques Roadshow.” Furniture with a deteriorated finish usually ends up in a city landfill.

2 ■ There are three broad approaches to refinishing: Make the furniture look new (as the maker or factory intended it to look); make the furniture look old but in good shape (usually with a satin or flat finish and maybe some glaze in recesses); or make the furniture look like something else (for example, change the color). All three are legitimate approaches.

Test for finish. To determine if an old finish is shellac or lacquer, dab on some denatured alcohol or lacquer thinner. Alcohol dissolves shellac. Lacquer thinner dissolves lacquer. I used alcohol here, so clearly, this is shellac.

3 ■ In most cases, the first step is to do as much of the wood repair, including regluing, as possible so glue seepage and minor damage occur on top of the old finish rather than on bare wood. Glue seepage and damage are then totally removed with the old finish. An exception is when paint has to be stripped first to see what wood or veneer exists underneath, so it can be matched.

4 ■ Stripping is messy, but it’s almost always a lot easier than sanding off an old finish or paint. Stripping also does less damage to the wood and to color that may be in the wood – color that is the result of the wood aging or a stain that had been applied.

5 ■ There are two broad categories of products that can be used to strip old paint or finish from furniture: paint stripper and the solvent for the finish. (Heat guns and caustic lye strippers are very risky on furniture because of the damage they can cause to the wood, veneer and glue joints.)

6 ■ There are five large categories of paint strippers. The primary differences are strength, speed and toxicity, which tend to correspond. From strongest, fastest and most toxic, the categories are:
■ High in methylene chloride (labeled “non-flammable” and noticeably heavier).
■  Low in methylene chloride (labeled “flammable” with methylene chloride listed as an ingredient).
■ Strong solvent, but no methylene chloride (labeled “flammable” with no methylene chloride listed).
Note: Each of the above contains wax that rises to the surface and slows evaporation. This wax has to be washed off the wood with a solvent or strong detergent after the coating has been stripped, or there may be bonding and drying problems with the newly applied finish.
■ Refinisher (very similar to strong-solvent, non-methylene-chloride strippers, but without wax to slow evaporation).
■ NMP (packaged in plastic containers with n-methyl pyrrolidone listed as an ingredient).

7 ■ Denatured alcohol will dissolve and remove shellac, which was used on almost all furniture finished between the 1820s and 1920s. Lacquer thinner will dissolve and remove lacquer, which was used on almost all furniture finished after the 1920s. Keep the solvent in contact with the finish using wetted rags or paper towels. When the finish has dissolved, wipe it off.

8 ■ It’s rare that much sanding is required after the finish has been stripped. Sanding with fine sandpaper is necessary only to check that all the finish has been removed. Any remaining finish will gum up the sandpaper, telling you that more stripping is necessary. Usually, the less sanding the better.

Finish residue. When stripping a finish, don’t be fooled by stain that isn’t removed. It’s rare that all the stain comes out of the wood. Instead, look for remaining shiny areas that indicate you haven’t removed all the finish.

9 ■ Don’t be tricked by remaining stain in the wood. No stripper will remove all the stain, and it’s not necessary to remove the stain unless you want to make the color lighter.

10 ■ Use oxalic acid, sold in crystal form as “wood bleach,” to remove dark watermarks. Dissolve the crystals to a saturated solution in hot water and brush the solution onto the entire surface. After the solution has dried back into crystal form, wash (don’t brush) the crystals off the wood. The crystals are toxic to breathe.

11 ■ You can use stain to create whatever decorative effect you want. But it was rare that pre-20th-century furniture was stained. The darker colors associated with most old mahogany and cherry furniture is the result of oxidation (age). Old walnut usually lightens a little.

12 ■ The most efficient method of applying stain is with a rag or spray gun. Wet the surface thoroughly, then wipe off the excess with a dry cloth.

13 ■ Most 20th-century mahogany, and some walnut and quartersawn oak, had its pores filled when the furniture was made. You need to repeat this filling using paste wood filler (not the same as wood putty), especially on tabletops, when you refinish or the wood will look too raw. Just as with wood putty, it’s best to use a colored filler because it doesn’t take stain well after it has dried.

Fish eye. It’s usually easy to spot the potential for fish eye by brushing the surface with mineral spirits, water or any stain. To avoid fish eye when the finish is applied, the silicone that causes the problem has to be washed out of the wood or blocked with shellac, or silicone has to be added to the finish to lower its surface tension.14 ■ It’s best to apply a thin sealer coat under the wood filler to make the excess filler easier to wipe off and to create a cushion that allows you to sand off any streaks you may leave without cutting into stain or the color of the wood below.

15 ■ A common refinishing problem is “fish eye.” The newly applied finish bunches up into ridges or crater-like depressions caused by silicone (which is found in most furniture polishes) having gotten into the wood through cracks in the old finish. Silicone is a very slick oil and isn’t fully removed in the stripping process.

There are three ways to deal with silicone contamination: Wash the wood many times with a strong detergent or solvent such as mineral spirits to thin and remove the oil from the pores; seal the wood with shellac to block the oil from getting into the finish; or add silicone (“fish-eye eliminator”) to the finish to lower its surface tension so it will flow out level.

Stripping with alcohol. Shellac can be removed by placing alcohol-soaked rags in contact with the finish for a short time until it’s easy to simply wipe off the finish. Lacquer can be removed the same way using lacquer thinner.

16 ■ The final step is to apply a finish. Shellac is a good choice if you want to be consistent with the original finish used on 19th-century furniture. Lacquer is much more versatile than shellac if you are spraying. Varnish, including polyurethane varnish, is the most durable of all consumer finishes. Water-based finish provides easy brush clean-up and reduced odor, but creates a “washed-out” look on darker woods unless you use a stain underneath the finish.

Oil finishes rarely look good on old furniture because oil finishes are too thin on the wood. PWM

Bob Flexner is the author of the new books “Flexner on Finishing” and “Wood Finishing 101.”

ARTICLES: We have many finishing articles available on our web site, free.
ARTICLE: Furniture prior to World War II was constructed using hot  hide glue. Read our recipe for “Liquid Hide Glue.”
TO BUY: Get Bob Flexner’s new book, “Flexner on Finishing.”

From the August 2011 issue #191
Buy this issue now