Michael Dresdner » compatibility

Spot on

Michael — Thu, 02 Dec 2010 14:00:52 +0000

Q: Can no-sand floor renewal finish systems fill in worn spots on a baked on factory finish?
A: Yes, they are made to go over any finish. However, the problem with worn spots is that they may or may not look the same. Many, but not all, finishes, both commercial and industrial, add amber to the wood. The refinishing kits are typically waterbased and do not add any color to wood. Thus, the bare spots may come out a slightly different color than those areas with original finish. It all depends on whether the original finish was slightly amber or water clear.

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.

Inked salad bowls

Michael — Fri, 29 Oct 2010 14:00:31 +0000

Q: I want to color walnut sapwood on salad bowls with India ink, then seal it with shellac and salad bowl finish. Would this be safe for food?
A: Yep. By the way, I would let the ink dry then go straight to salad bowl finish. In this case, adding shellac would weaken the finish.

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.

Michael — Wed, 15 Sep 2010 14:00:25 +0000

Q: I am finishing alder with Daly’s Benite and CrystalFin, spraying with a Fuji gun and #55 needle. I have been using water damp cloths as tack rags. I am getting white spots in the dried finish. Most areas are fine, but some of the areas look like white overspray. The spots are down in the cured finish. I did not apply a sealer. Could this be an issue?
A: Probably not. The most common cause of white in waterbased coatings comes from applying it too heavily, or working in conditions where the water does not have time to evaporate before the finish starts to cure. Try spraying thinner coats, work on days when the humidity is low, or add a slowing solvent to the finish.

Compatibility

Michael — Thu, 02 Sep 2010 14:00:15 +0000

Q: A friend told me that he used nitrocellulose sanding sealer on wood, and then finished it over with an automotive two part clear coat. Does this seem possible? To me it just sounds like the two different systems will not go together.
A: You are correct in worrying about incompatible systems, but as luck would have it, the two your friend used, while not ideal, are compatible. He’ll be fine as long as he did not pile on too much soft sanding sealer below the harder topcoat.

Acetone and the VOC question

Michael — Tue, 31 Aug 2010 14:00:47 +0000

Q: I found some conflicting information on thinning nitrocellulose lacquer. Some very good finishers use straight acetone while others write of using lacquer thinner. I realize lacquer thinner is a soup so there are many possible formulations, but generally I was wondering how acetone might behave differently from thinner.
A: Acetone, a ketone, is one component of lacquer thinner. Others include esters, glycol ethers, alcohols and possibly aliphatic hydrocarbons. Each component has a particular task, and in some cases, a mixture or variety will be a more effective solvent. A lot depends on the resins in the lacquer itself. The manufacturer of the lacquer will know what the best thinner mix will be, which is why some lacquers come with specific thinners likely to work better than the wide spectrum ones available in home and paint stores.
One trend in the industry is to reduce VOCs, either for green reasons or to placate EPA inspectors. Acetone is an exempt solvent, thus it is not, legally, a VOC. For shops trying to reduce their VOC output, using acetone instead of lacquer thinner is a useful strategy.
Generally speaking, acetone is a faster solvent than most lacquer thinner mixtures. It’s rarely as effective as a wide spectrum solvent. Thus, adding acetone alone will most likely change the way the finish behaves. Whether it is better or worse for your style of spraying and finishing needs is a very subjective question. Try it and see if you like it better. I should warn you, though, that some lacquers, such as those that require high levels of glycol ethers as their primary solvent, will not behave as well with an overabundance of acetone in the mix.

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.

Epoxy over shellac

Michael — Fri, 27 Aug 2010 14:00:02 +0000

Q: Can epoxy go over shellac?
A: It depends on the epoxy. Most contain either alcohols or amines, either of which will bond to shellac, but because epoxy formulas can vary so widely, it would be best to either know more about the formulation you are using, or do a sample followed by a cross hatch adhesion test.

Over pre-catalyzed

Michael — Sat, 21 Aug 2010 14:00:05 +0000

Q: Can you apply brushing lacquer over sprayed pre-catalyzed lacquer?
A: That depends on the pre-catalyzed finish, and how long it has been cured. In some cases you can; in others you risk cracking and wrinkling. Because it is so risky without knowing a whole lot more about the particular coating involved than anyone outside the formulator’s lab is likely to know, I would not advise it.