let's talk about lacquer

[B. Howard]

`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 it's 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 a water borne coating which simply put can not be a true air dry acrylic (which is what all lacquers actually are).There are pure nitro lacquers, pure acrylic lacquers, and modified lacquers which are somewhere in between. That just covers the actual resins.The differences are that 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". 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 emissions.

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 a large 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 the opposite is true to yield the same results. The addition of extra solvents is actually counter productive. 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. Even with a good, modern gun having the correct tip/needle 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 you gun set up again.

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 days 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. This old trick was used by cabinet and furniture finishers to take advantage of the ultra low surface tension it creates for use on an off the gun finish. Acetone is the most aggressive of the aromatics and since we will be level sanding and buffing anyway there is no need to get this brutal. Acetone will quickly go right down through all the layers of the coating already on the surface. It can cause problems with the pore fill, or stains and glazes that will not show up for several months.

Hope this helps provide a bit of guidance for those looking to spray there own work and if anyone has any questions please feel free to ask either here or through a PM or E-mail.

[Kevin Sjostrand]

A lot of great information there Brian, thanks for taking the time to splain some things.

Kevin

[tippie53]

thanks Brian for posting.
I like to see good information

[B. Howard]

I intend to keep expanding on some of the things I brought up in my first post. But before we do that I would be remiss if I did not bring up some other important topics.

Safety first! Air dry lacquers are dangerous. They are extremely flammable. Not just the product in the can, the spray fog is down right explosive. If doing this indoors an explosion proof fan should be mandatory! Lighting and other electric components should be 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 voc's is a must. These chemicals can also be absorbed directly through the skin so gloves etc. are a good idea. This stuff its a nuerotoxin, a carcinogen, and destroys liver and kidney tissue. Again, not just while spraying but while drying too. So I advise you do not do this in your home! You don't even want the things hanging in your home to dry.

Another topic we need to cover is compliance. A topic no one seems to talk about. As a pure hobbiest you have a lot of lee way. 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 an not an expert on these matters and if you need clarification you should seek one. But at the federal epa level there are two basic thresholds you need to stay under before you needed to do to 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 regs in large parts off the country are more stringent and will supersede this so you need to check on these things.

When i next get a chance we will start talking about the differences between nitro and acrylic and what that means us as instrument makers.

[tippie53]

I also use other solvents with this for retarding dry time and flow out. Nito is a great finish and one that has earned the place in your building. I have since moved to having this done. As a business decision it is easier and cost savings to sub this out.
A good finish guy will do this cheaper than you can do it yourself

[B. Howard]

Ok, so lets'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. True 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. 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 air dry 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 it's use for bindings and pick guards. Just as with those old pickguards, as it ages the resin brakes down and degrades. In the process it shrinks a bit, but it is no longer out-gassing. If it were it would be illegal to use for interior surfaces just like coatings 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 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/sunfade that nitros had. Acrylics are synthetic resins made from polymerized monomers and other fun things. They are true clearcoats or in the wood working industry parlance "water white". 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 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.

Then there are the modified lacquers. They are 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. And their properties will fall somewhere in between nitro and acrylic. 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. We are a small niche that prefers the old type nitro, or air dry lacquer at all. All of these lacquers can generally be repaired with the traditional drop fill techniques and will readily bond with each other.

[B. Howard]

Since we are talking about finishing wood, we need to talk about sealer. Wood is porous and will readily absorb the finish if not sealed. A good sealer will also provide enough build to be level sanded and will have additives to make that sanding easier. There are lot's of things that have been used.

De-waxed shellac is typically a very good sealer and has worked well over the years. It has the ability to not only seal the surface so the finish will not soak in but will also seal in stains, sap, knots and even oils and grease to some extent. The old adage was that shellac sticks to anything and anything sticks to shellac. Sounds like the perfect sealer, right? well for quite sometime it was but in recent years the chemistry of new coatings has evolved to a point where this is not always the case.Therefore a test should be done with your finish over the shellac to see if it will work. This should include impact testing as well as temp and humidity extremes to be sure of functionality. Typically shellac still works well under nitro and modified lacquers, acrylics can get to be a problem.

Nitro lacquer was always considered to be self sealing, and to some extent this may still be true but is generally frowned upon for several reasons.The old way was to cut the lacquer with a lot of fast solvent and spray a few coats to seal the surface and then keep spraying with uncut lacquer. This was the process that led to all those finishes with really high dry film builds that have cracked and crazed as time has passed. Not only is this bad from an environmental standpoint because of all the extra VOC's that are put out, it is not a very good method of sealing wood. Especially if you want to level sand between your sealer and finish.

Vinyl sealer is what most people think of as going with a lacquer finish these days. And if you are using acrylics or modified lacquers it is most likely the product you want. But caution must be exercised here. Not all vinyl sealers are the same. They are generally developed as part of a finish system. As such it is always best practice to use the recommended sealer from the same manufacturer otherwise problems arising from incompatible chemistry may result in film failure down the road. Another caution is if you are using a pure nitrocellulose lacquer, they do not tend to bond as well to a vinyl sealer. If you must use a vinyl under a pure nitro you should use it as more of a bond or tie coat than as a traditional sealer.

[Kevin Sjostrand]

Brian,
Can you shead some understanding on what Pre-Catalyzed Lacquer is, and how it compares to say, the Stringed Instrument Lacquer that Mohawk sells and some others like Hood's sells?
Thanks.

Kevin

[B. Howard]

Sure, now would be a great time to point out those differences.

the Mowhawk instrument lacquer looks to be a modified nitro type, based upon the recommended sealers. This is typically what is available at most retailers in most brands. I don't know what makes this product more suitable than one of their other finishes for what we do. 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, rubber or glass? This will effect us when we buff.

Now let's talk pre catalyzed lacquer. This is a basically an air dry type of 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 added to it. These are resin type compounds that will chemically lock the other resins in place. The reaction is triggered when enough solvent evaporates and the ppm of the cross-linker reaches a point where the linking reaction begins spontaneously. These coating 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 solidify in the fluid passages and require a break down and clean of your equipment. Interestingly, it appears that it is not always labeled as such. I was talking to someone who was using the new Cardinal nitro formulation and they asked me about it crusting up in the guns after a few hours....telltale sign it is a catalyzed product. 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.

[JLT]

Not all vinyl sealers are the same. They are generally developed as part of a finish system. As such it is always best practice to use the recommended sealer from the same manufacturer otherwise problems arising from incompatible chemistry may result in film failure down the road.

These posts are gold, Brian! Keep 'em rolling!

I have a question of my own. I use the LMI/StewMac "brown tape" to hold the bindings in place as I glue them. All too often, when I remove the tape from the top, a little of the soundboard spruce or cedar goes with it (it's almost a dead giveaway that I made the instrument). I've been told that the way to prevent this is to pre-coat the soundboard with shellac.

Here's the question: I've also been using the Behlen's Stringed Instrument Lacquer and the Vinyl Sealer for the instruments. Will pre-shellacking the top interfere with the adhesion of the sealer, or will I have to scrape away the shellac before I apply the sealer? Or will the sealer work as well to keep the spruce intact so I can skip the shellac? My main concern isn't the extra work involved (although I am a very lazy person in some respects) but the fact that if I don't get the shellac completely off, it may discolor or otherwise interfere with the finish.