Michael Dresdner » polyurethane

Jewelry finish

Michael — Wed, 24 Nov 2010 17:18:07 +0000

Q: What would be the best non toxic finish I could use on jewelry made from wood?
A: I’m not sure what you mean by non-toxic, as that is a term used typically for things we ingest.
If you mean that the finish itself is edible or drinkable in its liquid form, that would include raw linseed oil (but not boiled linseed oil), mineral oil, which is a wood treatment but not a finish, shellac cut into pure ethanol, which is potable but intoxicating, and any plant or animal wax, such as beeswax, with no solvent added to it. None of the above would make particularly good finishes for jewelry, for a variety of reasons.
Of course, pretty much any finish is non-toxic once it cures, since most are simply inert plastics once they dry, and we don’t generally have the ability to digest plastic. Eat it and it will pass right through you.
If, on the other hand, you mean non-reactive, meaning finishes that typically do not cause allergic reactions on the average person’s skin, that varies tremendously with the individual and what he or she is allergic to. Still, except for nut oils near people with nut allergies, I don’t know offhand of any finish that causes skin irritation, but admittedly, there are people allergic to darn near everything out there.
Personally, I would go with an oil based varnish or oil based polyurethane. They are durable, are not affected by skin oils, acids, bases or heat, and will not react or soften in contact with perfumes, skin creams or nail polish remover.

Over toning lacquer

Michael — Thu, 11 Nov 2010 14:00:06 +0000

Q: I used a toning lacquer to color the wood. I want to put polyurethane over it. Can I do this?
A: Yes, you can.

Bigleaf, big pattern

Michael — Tue, 28 Sep 2010 14:00:07 +0000

Q: I am making a cabinet door of solid cherry with a quilted maple panel. I want a bit of contrast between the cherry and maple and to show off the quilting in the maple. I was planning to stain the maple with a cherry color oil based stain and sand it lightly with 600 grit before applying wipe on polyurethane on the entire door.
A: Most quilted maple is western bigleaf maple, and unlike its eastern rock maple cousin, will absorb some pigmented stain. However, the ideal way to intensify grain in maple is to go with something that has a lower molecular weight. In the case of stains, that would be dye. A weak dye, flooded on and wiped off, will intensify the quilt pattern as well as adding color. Of course, an oil based finish, such as wipe on polyurethane, will add a slight amber color, which will also intensify the grain. In short, you don’t need stain to bring out the grain, though if your goal is to change the color to match the cherry, yet still pop the grain, dye is the way to go.

Auntie Oxidant

Michael — Fri, 24 Sep 2010 14:00:07 +0000

Q: I made some replacement handgun grips of cocobolo. Realizing that cocobolo has a lot of natural oils in it I first sealed it with shellac, then filled the pores with a product called z-poxy. I then sanded and applied one coat of Minwax polyurethane. Three weeks later I can make a thumb print in the finish. I think I sanded through my seal coat, because the poly is not curing. Can I remove the uncured poly with mineral spirits?
A: I doubt it, unless it is still fully liquid. What typically happens is that exposure to the raw cocobolo (where you sanded through) will inhibit the cure of an oil based finish, but not keep it entirely liquid. Thus, it stays tacky or gummy.
You’re certainly welcome to try scrubbing off the finish with mineral spirits on a coarse nylon abrasive pad, but in most cases you will have to resort to at least refinisher if not stripper. Since the parts are so small that should not be a big deal. You can also sand back to raw wood, again, since the parts are so small.
For the record, the reason oil based finishes don’t cure over dalbergias, such as cocobolo, has nothing to do with the wood appearing oily. In fact, it contains resins, but not, strictly speaking, oils. The problem is that the wood also contains a natural anti-oxidant.
Think of how citric acid, another natural plant anti-oxidant, slows the oxidation and browning of freshly cut apples if they are dipped in lemon juice. The anti-oxidant in dalbergia woods does the same thing.
Oil based polyurethane and varnish cure by oxidation polymerization; they take oxygen from the air and use it to link up small molecules to make large ones, thus turning a liquid finish into a solid film through chemical linking. The anti-oxidant is the culprit in delaying or preventing that chemical reaction.

Glazy days of summer

Michael — Mon, 30 Aug 2010 14:00:05 +0000

Q: I’ve stained and sealed with Zinsser SealCoat and plan to glaze with a Van Dyke oil-based glaze. I wanted to top with Zar Interior Polyurethane antique flat, but Zar says polyurethane will delaminate off either the glaze or SealCoat, which has no wax, but may contain stearates. Now what?
A: First, SealCoat contains no stearates and is perfectly compatible under both oil based and waterbased polyurethane. Second, oil based polyurethane will bond just fine to oil based glaze. The problem, and it has nothing to do with Zar or any other polyurethane for that matter, comes if you leave too much glaze on the surface, or if you mix the glaze with insufficient binder.
Whether a coat adheres to the previous coat is only half the problem. The other half is the integrity of the middle coat itself. If you create a glaze made of pigment with so little binder that when it dries, it acts like a barely held together layer of pigment powder, it will be the weak layer in your finish. Even if finish sticks to the top of this glaze layer, and it sticks to what is beneath it, if the layer itself breaks apart, you have delamination.
The key, then, is either to make sure you add very little glaze before sealing, and you can do that by glazing several times if need be with sealer between each application, or use a glaze that has enough binder to maintain its own integrity as a coat of finish.

Oil based lacquer?

Michael — Thu, 19 Aug 2010 14:00:04 +0000

Q: Can I put oil based wiping polyurethane over oil based spray lacquer?
A: The simple answer to your question is yes, you can put oil based polyurethane, whether wiped, brushed or sprayed, over almost anything, including lacquer. For the record, though, lacquer is, by definition, an evaporative finish, while oil based finishes are, by definition, reactive. In other words, the terminology indicates the process by which the finish goes from liquid to solid film. Thus the term “oil based lacquer” is at best a misnomer. While one could make an evaporative finish using a stable resin derived from oil, that should still be called simply lacquer.

Gel varnish, still hiding

Michael — Wed, 07 Jul 2010 14:00:28 +0000

Q: I have used the gel varnish that was sold at Rockler for a number of years with outstanding results. Since Rockler no longer carries it and there appears to be a company change taking place I am wondering if there is another brand of gel varnish you recommend.
A: That particular gel polyurethane was made by Lawrence McFadden and sold under several labels, including Rockler, Bartley and LMcF. The company was recently sold to Seagrave Coatings. Seagrave’s owner tells me there were a few problems with packaging and logistics that caused delays. You can expect to see the gel by early August in your favorite store. It will be sold under the Bartley label in both clear and colors.

Hungry rocker

Michael — Fri, 18 Jun 2010 14:00:50 +0000

Q: I just finished my fifth Maloof style rocker and have not been happy with finishing results yet. On the latest I just applied my second coat of Danish finish. I followed the instructions and wiped both coats dry. I want more shine; what can I put on top of the oil?
A: You have several good options. You can continue with more coats of Danish oil, though it will take at least a dozen before you get any noticeable sheen. You can also switch to wipe on oil based varnish or polyurethane, which will be just as easy and will build a bit faster. Or, you can go to a brush on varnish or polyurethane for the quickest build and highest shine.

No fade, no shade

Michael — Tue, 15 Jun 2010 14:00:10 +0000

Q: We are in the process of making a dining room table out of solid padauk and purpleheart, and wanted to know the best way to finish it. We would love to be able to keep the colors of the natural wood, without ambering over time. A clear, durable finish would be great as well.
A: For a clear durable finish that will not amber over time use a waterbased polyurethane. It goes on clear and will not amber. For more durability, use a cross linking waterbased floor finish. However, neither of those will prevent the sun fading the wood. For that you’d want an exterior waterbased finish that contains UV absorbers and blockers, but it will be softer and not as durable. Even at that, it will only delay the fading of the wood colors, not stop it entirely. Sadly, finishing, like life, is full of tough choices.

Fishy finish

Michael — Mon, 07 Jun 2010 14:00:06 +0000

Q: I stripped an oak fish tank stand and applied two heavy coats of espresso colored Varathane oil stain, leaving it sit one hour before wiping it off. I then applied two heavy coats of Watco Teak Oil and left them overnight. I found a sticky, gooey mess in the morning. When I tried to remove it using more Teak Oil, much of the color came off as well.
A: I am not surprised. Stain is meant to be wiped on and wiped off completely, leaving only what the wood can absorb. Layering it on heavily or leaving it longer won’t result in anything but trouble. Granted, more porous woods, like poplar, will absorb more stain than a dense wood like oak, and will get darker. The fact that stain came off when you scrubbed off the excess finish is an indicator that you left more on the wood than it was able to absorb.
The way around that is to use multiple types of color, not try to force one stain to override the nature of the wood you are putting it on. For instance, to get a very dark espresso, I’d start with a dark brownish black water soluble dye directly onto the raw, sanded wood, then follow that, when the dye was dry, with a dark wiping stain flooded on and wiped off. The combination will give you a dark, even color with wood grain still showing.
Teak oil is also meant to be wiped off in 15 minutes, not overnight, and letting it sit that long will indeed create a sticky mess. The simple answer is you really should read the directions and do what they say. At this point, you would be very wise to remove it all and start over.
Incidentally, Teak Oil, which is the exterior version of Danish oil, is not the finish I would have chosen in this case, for a variety of reasons that I have covered in previous posts. I’d have chosen an oil based polyurethane, either wipe on, gel or brush on, and I would go with several coats at least.