Air Dry Lacquers

AIR DRY LACQUERS

By Brian Howard

ORIGINALLY PUBLISHED IN  

"GUITARMAKER", ISSUE #89 FALL 2014

THE JOURNAL OF STRINGED INSTRUMENT CONSTRUCTION & REPAIR

PUBLISHED BY THE ASSOCIATION of STRINGED INSTRUMENT ARTISANS

Safety first! Air dry lacquers are dangerous. They are extremely flammable. Not just the product in the can, the spray fog is downright explosive. If doing this indoors an explosion proof fan should be mandatory! Lighting and other electronic components should be explosion proof as well. Once you are done spraying the hazard is not necessarily over, the fumes that build up from out gassing and drying can reach dangerous levels for several hours after you are done. Use proper care and caution, double check for sources of ignition like pilot lights etc. These compounds are also a serious health hazard. A properly fitting respirator rated for VOCs is a must. These chemicals can also be absorbed directly through the skin so gloves etc. are a good idea. This stuff is a neurotoxin, a carcinogen, and destroys liver and kidney tissue. Again, not just while spraying but while drying too. They will continue to evaporate solvent (off-gassing) for several weeks and perhaps months after sprayed. So I advise you do not do this in your home! You don't even want the things hanging in your home to dry. Disposal of unused material is a concern too, don’t ever pour it down any drain or other system that drains to fresh water. Fill a coffee can with kitty litter and pour the unused coating in it and stir it around. Let that set outside uncovered for a few days until it is solid and you can dispose of it with your regular refuse.

  Nitrocellulose lacquer has been around for almost 100 years. It is the grandfather of most modern sprayed coatings. For something this old there seems to be a lot of confusion as to its proper use. A lot of this arises from the generic use of the term lacquer and even to some extent "nitro" these days. I have seen the term lacquer used to apply to water borne coatings which simply put cannot be an actual lacquer as they are not able to be re-dissolved once dried or cured. There are pure nitro lacquers, pure acrylic lacquers. And modified lacquers which are somewhere in between, often a base resin like Nitrocellulose or polyester has other plastics added like styrene, urethane, vinyl, etc. As far as differences in actual base resins, actual nitrocellulose is made from the same process that makes movie film and modern smokeless powder. Acrylics are made from some type of synthetic resin generally made from acrylic acids yielding compounds like polyvinyl acetate. Alkyd coatings came next and are generally based on polyester and as such are not true air dry acrylics or "lacquers" as they actually cure by an acid based chemical process rather than actually “dry” and will not re-dissolve in solvents. There are also true urethanes but they are much more akin to epoxies than lacquers.

   So let’s talk more about the actual coatings and the types of resins involved, what the differences are and more importantly what does this possibly mean to us. First let's talk a bit more about Nitrocellulose lacquers. Pure nitrocellulose lacquers are made from nitrated cotton or other types of cellulose, though typically not wood pulp for some reason. They are considered "clear coats" but are actually not clear. They have an amber cast to them, different formulations may be more or less clear than others. They will continue to darken with age whether in a dry or a liquid state, getting quite dark after many years. This is mainly a reaction to UV light. They have a reputation for being able to be applied at an almost limitless thickness without problem. While this may appear true initially, problems develop later. Thicker film builds tend to crack and craze after some years pass. It is best practice to keep all dry film build with this product under 7 mils. This will help avoid problems with crazing. It also has a reputation for continuing to dry for years or even decades after it is applied. This too is false. The solvents are 99% evaporated from almost any finish in 30 days. Nitro lacquer shares a lot of basic chemistry with celluloid, one of the first thermoplastic type materials. I am sure we are all familiar with its use for bindings and pick guards. Just as with those old pick guards, as it ages the resin brakes down and degrades. In the process it shrinks a bit, but it is no longer out-gassing solvents from the application vehicle. If it were it would be illegal to use for interior products just like coatings or adhesives that contain urea forms. When applied to a surface such as a soundboard the coating (once dry) will increase the stiffness of the spruce. This effect will increase with thickness to a certain point. At some point either the additional mass of the extra coating or the coatings extra pliability due to thickness will start to decrease stiffness. This is another reason to keep those dry film builds down, according to a study done By Roger Siminoff  some time ago this peak is right around 6 mils.

   In the 50's acrylic lacquers were developed. This was driven by the auto industry. They wanted a coating that was just as easy to apply that did not have the problems with crazing and color shift from exposure that nitros had. Acrylics are synthetic resins made from polymerized monomers and other fun things. They are true clear coats or in the wood working industry parlance "water white". They go on clear and stay clear, they will not yellow with age. They also tend to be a bit softer than nitro. This makes them more abrasion resistant in use but also makes them a little bit tougher to level sand and buff. They can be used on wood with excellent results. CAB acrylics were the industry standard for cabinet finishing for a long time before catalyzed products came along. For our purposes they are perhaps a bit less desirable. Their slight increase in softness will help negate any stiffening effect they may have on our tops .The fact they are completely clear can make them look a bit more like “poly” rather than the warm nitro glow we are looking for.

Then there are the modified lacquers. They are typically nitro based with the addition of other synthetic resins. The goal was to provide the resistance to wear and crazing of an acrylic with the warm look and cheaper price of nitro. They come in an almost limitless variety with no two being exactly alike. They will typically have an amber cast and will darken with age. Most of the lacquers available to you over the counter will fall into one of the latter two categories, and most of what you will see labeled as stringed instrument lacquers will be a modified lacquer. We are a small niche that prefers the old type nitro, or air dry lacquers at all. I usually look at labels like instrument lacquer as being a marketing tool. I say that because it is a ready fact that any type of drying finish ( this rules out traditional wax only finishes) can and have been used on fine looking and sounding guitars. Taylor uses UV cure polyester and their guitars don't sound hateful. While I may not have mentioned it earlier, the main advantages of acrylic and modified type lacquers over nitro as far as most wood finishing is concerned is resistance to abrasion and chemicals. That first one is of interest to us. Most folks think that more abrasion resistant equals harder, wrong, it equals a softer coating. Think about it, which is easier to scratch with sandpaper, rubber or glass? This will affect us when we buff. Remember, the rest of the woodworking world does not typically level sand and buff their work. They are looking for an “off the gun” finish.

This discourse would not be complete if we did not touch on pre-catalyzed lacquers. This is a basically an air dry type of acrylic lacquer and is not really catalyzed in the same sense as a two part urethane or acid cure amino alkyd. It has a cross-linker and some catalyst added to it. The reaction is triggered when enough solvent evaporates and the ppm of the catalyst reaches a point where the linking reaction begins spontaneously. These coatings do not really have a set pot life per say but do have a definite shelf life. Most industrial pre cat is 90 days. I have seen some consumer grade stuff labeled at 1 year. While it has no set pot life, you can't simply leave it in the gun for days like an air dry. It will usually solidify in the fluid passages and require a break down and clean of your equipment. Pre cat lacquers that I have used were all water white clears. They can be re-dissolved like an air dry so they are still very repairable. That is a main difference between them and their post cat cousins. They are fast dry, generally considered 12-14 hours to stack and pack which makes them popular in a production setting.

   The solvent pack that is used as a vehicle for these resins has changed a lot in the past 100 years as well, heck it's changed a lot in the last 10. Solids content has gone up so that there are less pounds of solvent in every gallon. This is to lower VOC (Volatile organic compound) emissions. The types of solvents have changed as well. Most solvents fall into the category of VOCs, some of which have become highly regulated or outright banned. Limits on the amount Of VOCs in a coatings vehicle has forced some changes in what is in the same old looking can. Still other solvents fall into a group labeled HAPs (hazardous air pollutants) which are have been highly regulated and in instances banned for health and environmental reasons. You may be surprised to know that entire lines of water borne paints went extinct because of this as some of the heavy and water friendly solvents used in their manufacture fall into this category. Yes WB paints still have solvents in them.

  Another topic we need to cover is compliance. A topic no one seems to talk about. A pure hobbyist has a lot of latitude in what he can do (He is also limited to whatever products are available in his area over the counter). That all changes the minute you accept dollar one for your work. You are now a commercial user. As such you are responsible for complying with all applicable federal, state and local regulations. Ignorance will not be an excuse if someone comes by to see you and your permits etc. are not in order. I am not an expert on these matters and if you need clarification you should seek a qualified advisor. But at the federal EPA level there are two basic thresholds I am aware of that  you need to stay under before you needed to do too much about permits and such. First is daily output of voc must be less than 10 lbs. Annual use must be less than 55 gallons. Local regulations in large parts of the country are more stringent and will supersede this so you need to check on these things.

   All this leads to a lot of confusion as to what the proper way to apply these coatings is, especially today when we are more environmentally aware. I would like to tackle one of these misconceptions right off the bat. The notion that lacquers automatically need reduced in order to spray. That is false and most likely was never true. If and how much reduction is necessary will be determined by your equipment and your environmental conditions. Any finish schedule that contains info about how much to reduce the coatings should be treated with skepticism as that is never a daily constant in the shop. What I need to add in times of high humidity or temp will be quite different when it’s temperate or cold and dry to yield the same results. The addition of extra solvents is actually counterproductive. Thinner material takes more coats to get the same build (more labor involved), requires longer dry times, puts more VOC's into our air we breathe, and adds expense (quality thinners are not cheap!).

   So when should we reduce our coatings? Well the main reason is to adjust viscosity for our equipment. Siphon feed guns need thinner material to simply function. If you are still using one of these dinosaurs please retire it, gravity feed HVLP guns have gotten inexpensive enough that there is no excuse to use such old technology and create so much extra pollution. There is also a real benefit in material savings, modern guns have high transfer efficiency getting 75% or more of the material onto the surface as opposed to the 40% or so of a siphon cup. An HVLP gets more lacquer on the guitar and less in the filters or on the floor. That’s less money in material, less time applying it and less clean up, a win, win, win. Yet even with a good modern gun, having the correct tip/needle air pressure and fan pattern are essential. Higher solids coatings like are being used today require larger tips. I run a 25% solids through a 1.4mm tip, with no reduction. If you feel the need to add thinner to improve atomization, you should first adjust your gun. Modern guns are designed for modern coatings and will shoot them well right from the can as intended when properly set up. This is where a personal relationship with a local store can provide real benefit, they can help you pick the right tip/fan combination to get the best results with what you are using. If atomization is good but flow out is lacking, we can think about a bit of reduction. This adjustment should be well under 10% and preferably not more than 5%, if you need more than that make sure you are using a thinner with the correct evaporation rate and then check your gun set up again. When spraying small objects like guitars I am looking for a spray pattern about 4” wide with the gun 4”-6” off the surface.

  Most coatings manufacturers would rather you added a retarder instead of a thinner these days to simply slow the process without adding as many VOC's. This strategy works better for a large scale production type facility where they are mixing 100 gallons for a day’s spray than it does for us one pint at a time. But retarder is very useful as well. It you are getting a blush from high humidity or solvent pops from high temp with little humidity a bit of retarder in the mix will most likely fix you right up.

   Acetone is another one that comes up as an add to the spray mix. Acetone is the most aggressive of the aromatics. This old trick was used by cabinet and furniture finishers to take advantage of the ultra low surface tension it creates. This was very beneficial for an off the gun finish as is the norm in those trades.  But since we will be level sanding and buffing at the end anyway there is no need to get this brutal. While every time you spray a new coat the solvents in the new coating will soak through the existing film and soften it to some extent, acetone will do so very quickly.  This aggressive action can lead to problems with the pore fill, or stains and glazes that will not show up for several months.

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