Coating Over an Existing Finish: A Risky Business that Sometimes WorksPosted on December 3rd, 2012 By Bob Flexner No comments
Finishes deteriorate as they age. First they dull, then they begin to crack. Exposure to light in the higher ultra-violet ranges, such as sunlight and fluorescent light, accelerates the deterioration.
Finishes also get damaged from abuse, which can cause a surface to look bad.
In some cases, the only way to repair the deterioration or damage and make the furniture or woodwork look good again is to strip off the old finish and refinish.
There are many situations, however, where applying a coat or two of finish on top of what is already there can make the old deteriorated or damaged finish look new again. Coating over an old finish has the great advantages of avoiding the always-messy stripping process and greatly reducing the amount of work involved.
But applying a finish over an existing finish is one of the riskiest operations you can do in finishing because it can result in any number of problems. These include blistering, wrinkling, fish eye, orange peel and poor bonding.
Problems are difficult to predict, but there are three measures you can take to minimize your risk:
- Make sure the surface is clean.
- Make sure the surface is dull.
- Avoid using a finish that contains a solvent that could attack the existing coating.
It’s pretty obvious that a surface should be clean for a fresh coat of finish to bond well. There are four types of “dirt” that could cause problems: dust, grease, wax and silicone.
Dust and other common dirt can be wiped off the surface or washed off with water or soap and water. Grease is common in kitchens and can be removed with any strong detergent, TSP, household ammonia and water, or petroleum-distillate solvent such as mineral spirits or naphtha. Wax is a care product often applied to furniture and can be removed with any petroleum-distillate solvent without damaging the finish underneath.
Silicone contamination is common with old finishes and is a particularly difficult problem. Silicone is an oil that is contained in many furniture polishes and is difficult to totally remove from a surface because it gets through cracks in the finish and into the wood. Because it is so slick, silicone causes fish-eye (the finish bunching up in ridges as it flows away from areas, such as pores, where the silicone exists).
To a large degree, silicone can be removed by washing the surface with a petroleum-distillate solvent or with a strong detergent, TSP or household ammonia and water. But if the silicone has gotten into the wood through cracks in the old finish, you may not be able to get it all removed.
To deal with these situations, you can block the silicone fairly successfully by applying a first coat of shellac or you can add fish-eye eliminator to your finish. This product is silicone itself, and it lowers the surface tension of the finish enough so it can flow out over the slick silicone contamination.
In very bad cases, you may need to combine one or both of these remedies with a thorough cleaning to successfully overcome the problem.
Fish-eye eliminator is available from many distributors, suppliers and mail-order sources under several different names. One of the most common is “Smoothie.” Add an eyedropper full to a quart of finish and stir in. In particularly severe cases of contamination you may need to add more, but too much will cause a dullness in the finish.
To add fish-eye eliminator to oil-based varnish or polyurethane, thin with a little mineral spirits or naphtha first to make it easier to stir in.
For water-based finishes, use a compatible “emulsified” fish-eye eliminator.
All coatings have difficulty flowing out over glossy surfaces and “wetting” them well enough to establish a good bond, so it’s wise to scuff these surfaces before recoating. You can do this with any abrasive, including sandpaper, steel wool or Scotch-Brite. (Scuffing also helps with the cleaning by physically removing the “dirty” top layer.)
Don’t use steel wool if you intend to coat over using a water-based finish. Any steel shards left behind could rust and cause black spots.
Sometimes you can avoid the scuffing step by using a cleaning product that has the added effect of dulling the surface. Products that accomplish this include household ammonia and water, TSP and “liquid sandpaper,” which is a mixture of solvents (primarily alcohol) that both clean and dull.
The above explanations are fairly intuitive and pretty well understood by most finishers: a surface should be clean and dull in order to be coated over successfully. Problems caused by solvents aren’t so well understood.
If you think of the number of times you’ve successfully painted over an old clean-and-dull painted or finished surface, you know that coating over is possible. The reason the paint didn’t cause problems like blistering or wrinkling is that the solvents included in the paint were mild. The solvent that causes problems is lacquer thinner, which is included in nitrocellulose lacquer, CAB-acrylic lacquer and catalyzed lacquer.
In fact, any paint or finish that contains lacquer thinner or is thinned with lacquer thinner or with any of the active solvents in lacquer thinner (for example, acetone) has the possibility of attacking an existing coating and causing a problem.
Moreover, lacquer retarders, which evaporate much slower than standard lacquer thinners, are more problematic because they remain in contact with the existing coating for a longer time. This means that “brushing lacquers” are more risky to use because they are made with lacquer retarder. Also, the application of a thoroughly wet coat of a product containing lacquer thinner is more likely to cause problems than applying a thin or relatively dry coat.
So, what should you do if you want to use a product containing lacquer thinner to coat over an existing finish? With the goal of reducing the amount of time the solvents are in contact with the existing surface, here are some suggestions:
- Spray a couple of light “dusting” coats of finish and let them dry thoroughly before applying a wet coat.
- Avoid using lacquer retarder, even if this means waiting for drier or cooler weather. Also, don’t use brushing lacquer.
- Apply a barrier coat of shellac between the existing surface and the coating containing lacquer thinner. Shellac is thinned with alcohol, which rarely causes blistering or wrinkling. Be sure to let the shellac dry thoroughly before applying the product thinned with lacquer thinner, and apply this product lightly at first so it doesn’t stay wet on the surface long enough to dissolve through the shellac. Also, use shellac that is relatively fresh – that is, made within the last year or so, or it can be the culprit itself in causing the finish to wrinkle.
No matter what precautions you take, there is always a risk of problems. So you need to have a back-up plan of stripping and starting over, just in case.
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