Naphtha dries (evaporates) much faster than mineral spirits (paint thinner). This leads many to believe that thinning polyurethane with naphtha will make it dry faster. This is not entirely the case.
Like all varnishes, oil-based polyurethane dries in two steps. The first is evaporation of the thinner. The second, and much longer step, is by the crosslinking brought about by the introduction of oxygen from the air.
When you apply polyurethane, you notice that it stays wet on the surface for a short time as the thinner evaporates. Then the finish goes into a tacky or sticky stage for an hour or longer. This is the length of time it takes for the oxygen-induced crosslinking to occur. Adding a faster-evaporating thinner doesn’t speed this crosslinking.
What the naphtha (or paint thinner) does do, however, is thin the finish, so you’re brushing or spraying less solids. After the thinner evaporates, the thickness of the finish will be less, so there is less to oxidize and the finish will dry a little faster. The problem, though, as you have already guessed, is that you will have to apply more coats to get the same total thickness, so the total drying time may actually be longer.
Because different finishes have been used at different times, it’s often possible to date furniture simply by the finish on it.
In the 18th century and earlier, makers used whatever finish they had available, usually wax or linseed oil. If the maker lived near a port city, alcohol- or turpentine-soluble resins may have been available.
By the 1820s, transportation had improved and shellac flakes, along with other alcohol-soluble resins, became widely available. Alcohol evaporates rapidly so these finishes dry fast and don’t collect dust. As a result, shellac became the overwhelmingly dominant finish used on almost all furniture and woodwork for the next 100 years.
In the 1920s nitrocellulose lacquer became available and slowly replaced shellac as the favored finish. There were two primary reasons. First, shellac is a commodity product, so the higher the demand the higher the price. Second, in contrast to the single alcohol solvent required for shellac, lacquer thinner is a blend of many solvents with varying evaporation rates, so lacquer thinner can be adjusted so the lacquer dries at a normal rate in widely varying weather conditions.
By the 1960s, a number of additional resins had been developed, which began replacing lacquer. These include primarily very durable catalyzed, polyester and UV-cured finishes. In the amateur market, polyurethane varnish largely replaced alkyd varnish.
In the 1980s and 90s, water-based finishes were introduced to address the growing demand to reduce VOCs in coatings.
With this historical knowledge, and assuming you’re dealing with an original (not refinished) finish, you can fairly accurately establish the period during which the furniture was made by testing with various solvents using a process of elimination.
Wax dissolves in mineral spirits (paint thinner), naphtha and turpentine.
Shellac dissolves in alcohol.
Lacquer dissolves in lacquer thinner.
Water-based finish becomes soft and tacky in lacquer thinner, toluene and xylene.
Using one or more of these solvents, dab a little onto an inconspicuous area as shown in the accompanying picture of using denatured alcohol to test for shellac, and see what happens. If nothing happens with any of these solvents, the finish is varnish or one of the newer more-solvent-resistant finishes.
The sheen of a finish is measured as the reflectivity of the dried film – that is, the sharpness of an image reflected in the surface. Sheens vary from high gloss to very flat. The contrast between gloss and satin sheens is shown in the accompanying picture.
Finishes are supplied in various sheens, determined by how much flatting agent, which is usually silica, is added. These are usually identified with names such as gloss, semi-gloss, satin, matte, eggshell and flat. The names reflect the manufacturer’s interpretation and are often chosen for marketing purposes, so the actual sheens you get can vary noticeably among brands.
A more exact, and better, method of identification uses a numbering system from gloss (90 sheen) to flat (10 sheen). Some manufacturers, especially those targeting the professional market, use this system.
You can create your own customized sheen by mixing two sheens of a given finish type together, or by letting the flatting agent in one container settle, then pour off the top half into another container and mix the two parts.
To see what you have, you have to apply at least two coats. After sanding the first coat smooth, apply the second and let it dry. This will give you the accurate sheen.
The final sheen is established by the sheen of the last coat applied, not by an accumulation of sheens of all coats. So you can change the sheen on any surface simply by applying another coat with the desired sheen.
Sheen can also be achieved by rubbing the last coat with abrasives. The last abrasive used will establish the sheen, so you could go from satin to gloss, then back to satin and back to gloss. Once you begin rubbing with abrasives, the flatting-agent-created sheen of the finish you’re rubbing is no longer a factor. Satin can be rubbed to gloss, and gloss can be rubbed to satin.
The downside of a rubbed sheen is that it shows scratches more easily than a sheen created with flatting agents. Any light abrasion running across the ridges of the rubbed scratches flattens them and appears as a shallow scratch.
Sandpaper grits vary from very coarse to very fine. How do you choose which grit to use for any given situation?
The answer is actually quite simple. In all cases you are using the sandpaper to remove a problem. So choose a grit that removes that problem efficiently without creating larger than necessary scratches that then have to be sanded out.
For example, you would choose a coarser-grit sandpaper (#80 or #100) to remove severe washboarding caused by a jointer or planer but a finer grit (#120 or #150) on pre-sanded, veneered plywood or MDF. And you would begin sanding with an even finer grit (#180 or #220) if you were just checking to make sure your stripper had removed all the old finish from a refinishing project.
Likewise, you would choose a coarser-grit sandpaper (#220 or #320) to sand out brush marks in a finish but a finer-grit (#400 or #600) to remove fine dust nibs or mild orange peel.
In all cases remove coarser-grit scratches with finer grits until you reach the grit you want to end with. You can skip grits, but this will require that you sand a little longer with each grit than if you don’t skip grits.
These rules apply whether you’re sanding by hand, sanding with a pad or random-orbit sander, or sanding with a stationary sanding machine.
Commercial brands of paste wax are as good as anything you can make yourself, but you may still want to make your own just for fun or to get a specific color or shine. Here’s how to do it.
Grate the wax, or combination of waxes, into a container. Carnauba is available in flake form, so just put it in the container. Add turpentine, mineral spirits or naphtha in the ratio of about ½ pint of solvent to one pound of wax. Then put the container into a pot of water and heat it over a flame, stirring as necessary.
Never put the wax and solvent directly over a heat source; it can catch fire.
After the wax and solvent have combined and cooled, the consistency should be about that of butter in summertime. If you want it thicker, add more wax and reheat. If you want it thinner, add more solvent and reheat.
You can add rottenstone or oil- or Japan-pigment colorants to the wax while it’s still in liquid form and stir it in. Add enough colorant to make the color uniform. You can also add an oil-soluble dye by dissolving and thinning it first in naphtha or toluene.
The degree of shine or gloss corresponds to the melting temperature of the wax. Though the differences aren’t great, carnauba wax has a higher melting point, and thus a higher gloss, than beeswax, and beeswax has a higher melting point, and thus a higher gloss, than paraffin wax.
There are two large categories of random orbit sanders: electric and pneumatic (compressed air.) Both work well, though you need a fairly large compressor to power a pneumatic sander adequately.
Random orbit sanders are easy to use; you don’t need much instruction. But there are two general rules to keep in mind for achieving the best results:
- Let the weight of the sander do the work. If you press down on it, you’ll leave deeper and more obvious “squigglies” that then have to be sanded out. Simply move the sander slowly over the surface in some pattern that covers all areas about the same.
- Nevertheless, you’ll still leave squiggly marks on the surface. If they aren’t deep, they may not show up under a clear finish, but they will show through a stain. So it’s a good policy to always finish up by hand sanding with the grain using a flat block to back the sandpaper. Use the same grit sandpaper you used last with the sander. You can also use one grit finer, but you’ll need to sand a little longer.
Here’s a short history of shellac, some of it from my own experience.
Shellac was almost the only finish used on furniture from the 1820s to the 1920s when nitrocellulose lacquer was introduced. Shellac continued to be used widely as a complete finish by painters working in buildings until the 1950s.
By the 1990s the only remaining supplier of shellac in liquid form was Zinsser, which sells the shellac under the brands “Bulls Eye,” and “SealCoat,” which is a dewaxed variety. Zinsser specializes in sealers and primers, so it markets the shellac as a sealer.
The marketing strategy seems to center on getting influential magazine and book writers to recommend shellac as a sealer. This has led to shellac becoming widely identified as a sealer rather than as a complete finish.
Too much so, in my opinion, both because shellac still makes a great complete finish just like it used to, and because there’s no reason to use shellac as a sealer unless there’s a problem you need to seal off. The four common problems are:
- Silicone contamination in the wood from the use of certain furniture polishes. This contamination causes the finish to fish eye, which shows up as ridges, often in the shape of craters.
- Oil or wax on the wood. Shellac bonds better than other finishes to oily and waxy surfaces.
- Odors in the wood – for example, from smoke damage or animal urine. Shellac blocks these fairly effectively.
- Resin in the wood, especially from pine and other softwood knots and in certain exotic woods that grow in jungles. Examples include teak, rosewood and cocobolo. Shellac also blocks this resin fairly effectively.
Notice that except for the last one, the problems are typically refinishing problems. Also notice that all the major writers on finishing are, or were at one time, professional refinishers. So shellac was a very useful tool for them (us).
But shellac is not necessary if these problems don’t exist. And this is the point of this short history. No furniture or cabinet manufacturer uses shellac as a sealer. Only woodworkers who read the magazines, or are told by someone who does, think they should always use shellac as the sealer.
Usually that phrase is a warning. It can also be a really good idea.
In recent articles, I have written about the importance of eliminating bad habits in your finish program and how to replace them with better ones that are duplicable. But, how do you really know HOW you are doing? The proof is not always totally in the pudding, the finish. Sometimes you can end up with a good finish in spite of a horrible experience getting there, solely on the virtue of good gear and materials. The products can be better than you. Ideally, you want to be in charge of your finishing destiny.
We take those lucky days and run – gifts from the Finishing Gods. Personally, I would rather have a slight flaw in a finish where everything went right. I’m not so sure it is better to be lucky than to be good in the long run. Most of us prefer to be good, it is more predictable.
Most of the best finishers I have ever known are students of the game…always engaged in learning. One of the best ways that we learn new skills (or improve on existing ones) is by either doing them ourselves, or observing others in the act of doing them.
That certainly holds true for improving in sprayed finishes. I have spent countless hours observing others both on video and in person. Spraying, like many other skills, lends itself well to visual learning and assessment.
In recent years, my shop mate, Todd Pudvar, and I have made it a practice to video record most of the important spray projects that we do, and we even archive the videos on Youtube for other practitioners to learn from.
We have come to appreciate the value of being able to observe ourselves in action. We figured out that we document in writing most everything else related to a project – wood species, product used, spray equipment, tip sizes, settings, even climate info – so it only makes sense to go the extra step and commit to visual record for later review.
This is very easy to do. Even an iphone can get you started. That is how we started, then we upgraded to a nicer camera, tripod system, video card archiving, etc. I recommend avoiding the GoPro type of video for finishing. It lacks focus and clairty, and is limited to the perspective you already have – your own. I prefer to set a tripod from the angle that casts the most unfavorable light on the situation…for instance, the camera positioned to look across what you are spraying, and into a wash of natural light. That tends to show everything. I want to see the worst case scenario of all aspects of my program. The best aspects tend to take care of themselves, need little improvement, and only fool us into thinking we are all set.
What to Look For
When we are spraying a finish, we tend to be more focused on the finish as it relates to the surface being sprayed. This is partly because we know that our equipment is good, if we find ourselves having to focus on the gear instead of the result being created, that would be a bad thing. In fact, that would be a good time to stop spraying.
I have long taught others, in all styles of finishing, that only thing standing between the tools/products and the desired result is…YOU.
Some of the obvious:
- The pace at which you spray
- Your distance from the target
- The nature of your passes
- Perpendicular technique
- How you enter/exit a pass
- Trigger engagement and release
Each of these could be a chapter in the book you should be writing yourself about how you spray, so that when you are old and your eyesight is failing, you can reference it. Yes, that is why we all talk about “feel”.
Some of the more complex technical aspects that you should observe:
- Bounce back
- Pattern winks
- Your overlap and how the product lays into itself
- Your general comfort level as you work
These are qualities that are really hard to assess when you are “in the moment” as a finisher. But all of the items on both of the above lists in some way leave an impression on your finished product, for better or worse.
Many of us work in solitude, with little feedback from co-workers on projects. That makes it especially critical to get that additional perspective that a tripod camera shooting you from a different angle can give.
I am fortunate to have a shop mate and fellow finishing expert to work with daily. We are constantly observing ourselves and each other, and offering immediate feedback so that adjustments can be made before bad habits set in. And we still go back and study the videos. You can even go so far as to dump the video file into a movie maker program and slow the speed down for deeper analysis. Why not? Professional athletes do! How else would a baseball star find a hitch in his million dollar swing?
In our case, video analysis is not always to find issues in our technique. More often, it is related to trouble shooting the system we are working with. If we are using a certain HVLP with a certain finish and not getting that certain result that we certainly remember getting every time previously, we go digging into the archives – both the written and the visual documentation.
Almost every single time, we find the problem and correct it. Sometimes it requires a wholesale correction, as in the case where unbeknownst to us, that favorite clear finish we have always had great results with has been reformulated and we didn’t get the memo. That leads to either a “redial” of the product, or a change of product altogether.
Remember: Finishing is a Mind Game
Those who leave it to chance end up with dusty spray rigs sitting on shop shelves.
I know that I have written here about the Psychology of HVLP and the Fear of Finishing. Those matters are very real and always bear repeating in any healthy consideration of improvement as a finisher.
We all need to remember that our minds play tricks on us during the act of finishing. You can do everything right, and have it all go wrong…with a half wet piece sitting in front of you. Confidence and mental clarity are key.
Finding the quickest and safest alternate route to a temporary destination so that you can salvage the piece is the definition of disaster recovery for finishers. As long as the gun is moving material, not clogged and not spitting gunk all over the surface, it can be salvaged. Unless it is the final coat, which is an entire psychology unto itself. This is why we emphasize “duplicable practices”…treat every coat as if it is the last.
The point is, at that critical moment when you have done everything according to your documented experience with your chosen spray rig and product, and the fan lollygags out the gun and says “meh, not so much…”, your mind can almost trick you into thinking: “Ok, that’s how the fan was last time, let’s just do this and get it over with.”
These are things we try to tell ourselves to make ourselves forget how good it needs to be. Psychological corners are the easiest, but most costly, to cut. That is why we call ourselves on it through the use of video. We will go pull up the previous project, with that spray rig, set up the same way, in the same product, prepared the same way…and we will usually see a glaring difference and figure out the necessary adjustment.
If you really want to be proactive, study the video the day before so that you already have in your mind what you are looking for. Sports psychologists call it visualization. We call it common sense.
Watch yourself, and learn.
Stripping veneer is no different than stripping solid wood, with a few exceptions.
First, avoid using lye or a stripper that contains water. Because of the thinness of the veneer, these might work through and loosen the glue bond. Solvent-based strippers shouldn’t cause any problem that didn’t already exist.
Second, if you sand after stripping, which is usually a good idea to check that you have removed all the finish, do so lightly and with fine sandpaper. Otherwise, you might sand through, and a sand-through is almost impossible to fix.
I recommend doing this sanding by hand with just your fingers backing the sandpaper. Using a hand-held sanding machine is more risky because it is more aggressive and you can’t see what’s going on under the sander. Using a flat block to back the sandpaper, even on supposedly flat surfaces, can also lead to sand-throughs if there is any unevenness, which is common on old surfaces.
Glaze is very effective for decorating or highlighting grain, especially in large-pored woods such as oak, ash and elm (as shown in the two accompanying pictures).
The term “glaze” refers to a specific product, which is essentially a thickened stain. In addition to a pigment colorant glaze contains a binder (oil, varnish or water-based finish) to bind the colorant to the wood, and thickeners to prevent the glaze from running on vertical surfaces. On flat, horizontal surfaces you could use a pigment stain instead of a glaze.
Glaze, or stain used as a glaze, is always applied over a sealed surface so it doesn’t color the wood. The glaze is also always coated over by at least one coat of finish to keep it from being rubbed off and to bring out its true color. In other words, the glaze is sandwiched between coats of finish.
In the two examples pictured, the wood has been stained, then sealed with the first coat of finish, then glazed with all the excess not lodged in the pores wiped off, then topcoated.
You can also apply glaze over unstained wood, of course, to highlight the grain.