SEALERS FOR AIR DRY LACQUERS

SEALERS FOR AIR DRY LACQUERS

by

BRIAN HOWARD

   Safety first! Air dry lacquers and sealers 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 electric 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 nuerotoxin, 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.

   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 (it’s called sanding sealer for a reason). The sealer is really where the rubber meets the road as far as finishing is concerned. While sealing the wood so the finish doesn’t soak in is the main perceived function of these products they have some much more important functions. In addition to simply sealing they must grip steadfastly to the wood. And if one truly considers the cellular construction and hygroscopic nature of wood, this is not an easy task. And on the other side it must do the same with the resins in the finish, which are nothing like wood. It must form a connection of some sort between the two.

   The sealer is the workhorse of a wood finishing system. Most finish film failures are of a delaminating nature, where the film peels or pops apart. The majority of these failures occur on one side or the other of the sealer. This doesn’t necessarily mean your guitar is shedding like a reptile. These failures can also appear as hazy dots, spots and streaks inside the finish. These are referred to as abridgements, where the finish delaminates at a spot but the coating does not actually rupture or peel. It fails because the bond breaks for one reason or another. A major cause of this is improper prep work prior to sealing which causes the sealer to let go of the wood. Another common cause is incompatibility between the sealer and the finish or bad prep work between the sealer and the finish. This allows the finish resin to separate from the sealer. Sealer problems are not easy to fix and often do not really manifest until sometime well after the finish is applied. Often they will require the instrument be stripped and completely re-finished in order to rectify.

   Unfortunately, as with all other things in wood finishing, there is no single product that works in all situations. There are lots of things that have been used for this purpose through the years. Some finishes are totally self sealing and can be applied directly to bare wood. Shellac and drying oils like walnut oil or tung oil are examples of traditional self sealing finishes. Traditional varnishes may or may not be self sealing. Sometimes a coating is cut 50% or more with a reducer to create a sealer. More modern coatings may need a specifically engineered sealer such as a vinyl. If you are using a modern coating the best thing to do is use the sealer recommended by the manufacturer. Most consumer products have this info right on the label. With professional products you should obtain a copy of the technical data sheet (TDS) and follow the guidelines there. You should also obtain a Material safety data sheet (MSDS) for any products like these in your shop too.

   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 and provide a good bonding surface, 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? For a long time that was the case when the only other thing going that needed a sealer was spirit varnish or something like milk paint. Pigmented shellac was used very commonly as a primer to seal in knots when painting. The modern recommendation is to use de-waxed shellac for this purpose. That is good advice but not always true, some traditional finishes will work quite well whether the shellac is de-waxed or not. Most nitro and modified lacquers will work well over shellac. Some modern acrylics and alkyds may not. Even if the coating of your choice will work over shellac if the manufacturer has a specific product they make for use with it, that’s going to be your best bet. Manufacturers spend a lot of time engineering their coatings systems for the best possible results. The one drawback with shellac is it does not provide much build for level sanding and does not sand very well, usually gumming up sand paper rather quickly..

   A case of possible incompatibility with shellac is water borne coatings. While some manufacturers will tell you it is OK to use their product over shellac I only know of one that sells shellac specifically for use under their finish. And it is notably a water borne shellac, I am not completely sure how that works as shellac and water are typically at great odds with each other. That is where the problem comes in. In this instance the resins in the water borne coating will need to form a mechanical bond with the shellac relying mainly on gripping the surface texture , or “tooth” left by the level sanding operation. Even de-waxed shellac is far from water proof. So you create a surface for the resin to grip but when you spray the coating the water in the vehicle will soften up the shellac and some of the texture will flow out reducing the bond between the two. For that reason I personally do not recommend using shellac under any water bourn coating.  A lot of water borne coatings are self sealing anyway so why bother with the extra product when that is a better choice?

   Just because a coating is self sealing does not always make that the best choice though. 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. This is bad from an environmental standpoint because of all the extra VOC's that are put out. It is also not a very good method of sealing wood, especially if you want to level sand between your sealer and finish. At this point it becomes somewhat important to know what kind of lacquer you are using.

   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. If however you are using a pure nitro lacquer this could be a very bad choice.  Pure nitro does not typically perform well over such a flexible foundation.  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. Vinyl sealers were developed for use with coatings that have some type of plastic as part of their resin system. These sealers usually give good build and are easy to sand, both great qualities in a wood sealer. 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. Vinyl sealers tend to not work real well for repair or touch up work on older instruments. The extra plasticizers tend to swell up the old lacquer and leave it soft for a long time.

   Nitrocellulose sanding sealers are still quite common and available. Most of these are sterated, which means they have an additive to make them easier to sand and less likely to clog up sandpaper. This was the modern replacement for shellac and in a lot of instances a big improvement. This is the preferred sealer if you are using pure nitro top coats. These sealers are also widely used and recommended under poly-urethanes and other alkyds. These sealers have a milky white appearance in the can after being stirred, but dry clear with just a bit of amber cast to them. As far as I know they can be used under any of the lacquers we have discussed but not necessarily the best choice for some. These are also a good choice for repair and restoration work on lacquer guitars.

   One must be careful in the application of sealer. It must be remembered that the film thickness of the sealer becomes part of the overall thickness of the finish. Sealer is typically not as hard and durable as a top coat. And while a necessary part of the finish system if it is allowed to be too thick can cause its own problems. Ideally dry film thickness of the sealer after sanding should be under one mil. The sanding of the sealer to prepare it for the top coat is important as well. While most of the finishes we are talking about will actually bond molecularly to the sealer forming one continuous film some prep is needed to facilitate that. It is also highly desirable to give a completely level surface to start applying our top coats, any texture in the surface here will print through the clear coats. I dry sand all my sealers with 320. Whether or not you seal prior to pore filling (when needed) you want a coat of sealer over your pore fill to give a solid, even foundation for your finish.

  As you should have noted by now, I always recommend using the sealer recommended by the coatings manufacturer for best results. But invariably it always comes up and people want to know can I Use finish “A” over sealer “X”. The answer almost always is that no one really knows the answer to that, not even the coatings manufacturer. Once you are no longer using the components in the finish system together as designed you are pretty much on your own. Even if you find someone who says yes they have done that, unless you follow their procedure exactly you may not get the same results. And unless those results have been thoroughly tested it is still a bit of crap shoot. Finish failures are no fun to fix and will have a big impact on your reputation as a builder and the perceived quality of your instruments. So why take a chance? The finish is what people see as it is the last thing done, that’s why it’s called “finishing”. This is no place to pinch pennies or cut corners. Saving a few bucks using shellac or something else instead of the correct sealer is no bargain when the guitar is out in the wild and the finish fails in a customer’s hand.

   For those who must step outside the bounds of working within a system as engineered by a manufacturer you owe it to yourself to test your finish schedule before using it on anything of value.  A series of test panels should be made and subjected to testing for adhesion, abrasion, impact, UV exposure, Temperature and humidity extremes, etc. to verify functionality of the coating system as a whole. Testing finishes for adhesion and survivability is a complex subject and would make for its own series of articles. For those looking for more on this subject I would suggest starting with an article by Phil Stevenson published in Wood Digest’s Finishing, December 2003.