Michael Dresdner

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.

Q: I learned the hard way that a thin plastic cup I had could be dissolved by lacquer thinner. Can you tell me what kind of plastic container will not react and how I can tell a container is made of a safe plastic?
A: Nalgene, nylons, polyesters, acetal, polyethylene and polypropylene all have decent resistance to lacquer thinner. I gather you are using recycled food containers, like yogurt cups and the like. You can do a lot of research and figure out which plastic is being used based on the recycle codes http://www.earthodyssey.com/symbols.html or simply do it the quick and easy way; test your container first with a bit of thinner.

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.

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.

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.

Q: I am building my 10th guitar and am using epoxy over bare Indian rosewood to fill the pores, with great success. Can I also apply epoxy to the soundboard to eliminate “spruce ripples” and also fill voids in the rosette?
A: You can, but I would not advise it, though it might make an interesting experiment for a few guitars. The problem is that softwoods, like the spruce used on guitar tops, tend to absorb a good bit of impregnating coatings such as epoxy.
The beauty of spruce is that it has a high stiffness to mass ratio. In other words, it produces good tone because it is light, but stiff. Adding epoxy to the wood will increase its mass, and is not likely to add to its stiffness. Thus, it is likely to result in a slightly duller sounding top.
That said, I do know of a number of people who use polyester as a curing filler, even on spruce. It is admittedly much more brittle than most epoxies, and therefore likely to be a bit less problematic, but I’d want to hear a dozen identical test guitars, half with and half without curing filler on the tops, before I’d be convinced there was not some loss of tone in the process.

Q: Can I thin enamel for spraying, and if so, what solvent should I use?
A: Yes, but you will have to read the label to find out what to use. Because the term enamel is used for many types of coatings, you must know what type you are using to know the thinner for it. (See the previous blog post for a fuller explanation.) Most cans of enamel will tell you what you can thin it with. If not, it can often be thinned with the solvent suggested for cleanup, such as lacquer thinner for lacquer enamel and mineral spirits for oil based enamel. Be wary, though; waterbased coatings are an unfortunate exception. They clean up with water but my not tolerate thinning with it. In some cases they will not tolerate thinning at all; in others they will, but usually with something other than water.

Q: How does enamel differ from lacquer?
A: The term lacquer refers to how a finish dries. Specifically, it means a coating that dries solely by solvent evaporation. It does not give any clue to what resins were use in it, though of course, certain ones are particularly common.
The term enamel has no definitive meaning in coatings anymore, though it did at one time. Today, the term is simply used to mean a very durable paint, without giving any indication whether it is reactive, such as an oil based enamel, or evaporative, such as a lacquer enamel, or a multi-phase coating that has elements of curing methods.

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.

Q: You make a distinction between lacquer and oil based finishes. Why does it matter?
A: Lacquer, which is a category that includes shellac, is an evaporative finish. It cures solely by the solvent evaporating. That means the molecules floating around in the solvent when it is in its liquid form are exactly the same once the solvent evaporates. The film is formed by these long, gangly molecules twisting and clumping together, but not bonding chemically. Technically, they form an amorphous solid.
Because of that, evaporative finishes have two very distinct characteristics. First, they will redissolve with their own original solvent; lacquer with lacquer thinner, shellac in alcohol. Second, when subjected even to fairly low levels of heat, they start to liquefy. Hence, they are known as having poor solvent and heat resistance.
Oil based finishes cure very differently. They actually go through a chemical change in which small molecules link up and form larger molecules, eventually forming a solid. Though most do, an oil based finish does not need to have solvent in it to work. A good example of a solvent free finish is pure boiled linseed oil. Although no solvent evaporates, it goes from liquid to solid, using oxygen absorbed from the air to bond its molecules chemically. Epoxy, conversion varnish, polyester, UV coatings and even some types of polyurethane all bond through chemical reaction rather than solvent evaporation alone. Such finishes will not dissolve in their original solvent, and typically are thought of as having much higher chemical and heat resistance.