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| Wood species can be a factor when deciding which finish is appropriate. Michael gives our reader the lowdown on finishing cocobolo. |
Q: I'm making a flute case for my daughter out of cocobolo and was wondering what kind of finish I should use. I was told that some finishes are not compatible with this wood.
Christian Bradley
Bellevue, Nebraska
Michael Responds:
That's true. Cocobolo is a Dahlbergia, a member of the rosewood family and a group of woods that contain a natural antioxidant. As a result, they interfere with the drying of finishes that cure through oxygen polymerization, including most oil-based finishes. Coatings that cure through solvent evaporation, such as shellac, lacquer and water-based coatings, will all work. Of these, an exterior water-based polyurethane is your best bet.
Were it not for the wood restriction, my first choice for a flute case would have been an exterior oil-based polyurethane. It has the ability to withstand heat, cold, wood movement, water, abrasion and all manner of chemicals. Fortunately, there is a way around the antioxidant problem. Seal the case first with one coat of Zinsser SealCoat™, a dewaxed shellac-based sealer that is compatible under oils and will act as a barrier to the antioxidant. When the SealCoat is dry, apply several coats of exterior oil-based polyurethane.
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| Which filters are best for the different solvents used most commonly by woodworkers? If you know how to read them, the answers are printed right on the filters. |
Q: What types of cartridges and filters should I be using in my respirator? I use the typical solvents and finishes most woodshops use.
Jeff Damon
Wasilla, Alaska
Michael Responds: An "organic vapor cartridge," which is called just that by manufacturers and vendors, will handle the common solvents you are likely to use, but to work well, it must be paired with a pre-filter to stop airborne finish particles and dust. Most companies offer three types of pre-filters, designated "P" for oil-proof, "R" for oil-resistant and "N" for not for oil. If you do not spray oil-based coatings, you may be able to use the slightly cheaper "N" grade, but for wide-spectrum protection, your best bet is a "P"-type pre-filter atop an organic vapor cartridge.
Q: I made a coffee table using ash for the top, poplar for the aprons and pine for the legs. I applied Olympic Red Mahogany stain, and it was perfect on the white pine legs and poplar apron, but it is not dark enough on the ash. I've applied three coats, waiting as long as possible before wiping off, and it doesn't look any darker. I can make it almost dark enough when I apply a generous amount, but I think it's just lying on top. I plan to use oil-based polyurethane for the topcoat. Can this piece be saved?
Allen Freeman
West Boylston, Massachusetts
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| When attempting to match a stain color on different wood species, there are some great products that can help you out. |
Michael Responds: You were wise not to heap stain on and try to seal it in, as that can result in the finish delaminating later. As you now know, not all woods take stain the same way, even when they are sanded the same.
Fortunately, there are two common materials you can use to add extra color after staining: glaze and toner. Glaze is slower-drying stain modified with extra resin and is designed to go in between coats of finish. Toner is simply tinted finish. You can buy either in ready-to-use form, but in this case, you might want to mix your own.
Convert your oil-based stain to glaze by adding about an ounce of boiled linseed oil to a pint of stain. Apply it after your first coat of clear finish. Manipulate the glaze with a dry brush to apply a very thin, even coat of color, and let it dry at least overnight before applying the polyurethane. If you prefer toner, make it by adding up to two ounces of stain to a pint of oil-based polyurethane or varnish. Toner must be applied very evenly or it will show colored lap marks. Whichever route you choose, try it first on scraps of stained, sealed ash, since both techniques require some skill.
There are also two things you could have done to make the ash stain darker. One is to stop sanding at a coarser grit than you used for the pine and poplar. For example, stop sanding the ash at 150-grit, but continue to 220 for the other woods. Second, raise the grain of the ash, and only the ash, prior to staining. Raising the grain leaves the pores of the wood more open, and hence prone to absorb more stain. Clearly, this advice violates the first rule of finishing preparation: Always do the same thing to all parts of a piece. However, there are exceptions to every rule.