The Art of Finishing
Often I get emails asking me for advice on particular finishing situations. Many times, I can reply with a fairly specific answer. In other cases I can provide an answer only as a guideline with the caveat that there is a point where experimentation and common sense must take over.
I can’t tell you what is the “best” brand of a particular coating. I can’t tell you how much to “thin” your coating. I can give you known and established guidelines and I can make suggestions for you to consider in formulating your best end result.
This is because with finishing there are many variables. To note just a few: the work environment plays a big part. Temperature, humidity, etc. Next is the coating or finish. Solvent or waterbase? Clear or pigmented? Each individual brand has its own character. It is impossible for any one person to know the performance characteristics of every brand. Then, you have application method. Is it manual or are you spraying? If you are spraying, what technology or equipment are you using? I could go on and on.
Let’s look at some of the variables and understand their role in the finishing application process.
- Temperature: It is generally accepted that 70 degrees F is the optimum temperature to apply a finish with a humidity level of 45%-50%. It is generally accepted that working below 65 degrees F or above 85 degrees F with either too high or too low humidity can cause a finish to either dry too fast, too slow, or sometimes not perform as desired.
- Each brand of finish is formulated by a manufacturer to perform as they choose. In this case, Brand A might differ slightly from Brand B even though they are in the same finish category. For example: two different brands of lacquer or two different brands of a waterbase finish might “behave” slightly differently upon application based on the formulation. That could be drying time, flow out or leveling ability, etc.
- Application method also plays a role in finish application. If you are applying a finish manually, are you brushing, dipping, rolling, wiping, spraying, etc.
- If you are spraying what technology are you using? Air compressor, HVLP Turbospray, Air assisted airless, etc.
- If you are spraying, nozzle, needle and air cap size is relevant. What’s the best combination of the three for your particular equipment and the coating of choice for this particular application.
Do you see what I mean that it’s always not so easy to supply an answer in “black and white” (no pun intended). There are many grey areas including each individual’s style of finishing and one of my big words “expectations.” What are you looking for in your end result?
If you read different experts and their recommendations on finishing methodology you will see many diverging opinions on how to complete various finishing tasks. This is no different for me. I consider my expertise to be HVLP Turbospray technology with additional knowledge of coatings and finishes and how they interact with HVLP Turbospray. Yet, I’ve seen writing on this subject that can be different from the information that I teach and write about.
So, which expert is right? Most of the time, we can say almost all since each is providing information based on their experiences and knowledge.
This is why I encourage learning, listening, experiencing and experimentation until you reach what you consider to be the perfect finish that meets your expectations.
My words of wisdom for this letter:
Try different brands or types of coatings or finishes so that you can form your own opinion. I guarantee that you will find one brand that suits your needs based on appearance, ease of use, etc.
Learn as much as you can from the experts. Talk to other finishers. Read what experts have to say. You will soon learn whose advice you are comfortable with and use their ideas and guidelines to form your own opinions. Woodworking is an art. Finishing is part of the art. Just as an artist has a variety of tools, paints, and techniques, so does the finisher. How your finished product looks is up to you.
This is why I always preface my comments to say “I am giving you guidelines.. It is up to you to take the information and use that information to your advantage. I encourage experimentation to learn what works for you.
Sr. Vice President, Sales & Marketing
Apollo Sprayers International, Inc.
Product of the Month: Aqua Coat Water Based Gel Stains
Aqua Coat water based gel stains come in almost every color imaginable. Some advantages to using Aqua Coat gel stains:
- They won’t run even on vertical surfaces, chair legs etc.
- No light or dark areas, or blotchiness
- Water based gels are easy to apply
- Good control over color and depth.
Using a clean rag, gel stains can be applied much like using a paste wax or shoe polish. To avoid lap marks when working on larger areas, try applying gel to smaller sections. When working on larger areas apply gel to smaller sections keeping a “wet edge” between each section. Gels are very easy to apply and you have good control over color and depth. Apply a liberal amount of gel and rub into the wood surface, removing any extra as you go. Water based gel stain can be re-applied for a deeper color, shading, or even used as a glaze. Take extra care in getting excess gel out of nooks and crannies, inside corners, etc., to avoid dark spots. Re-coat with additional gel stain, or a waterbased topcoat. We recommend using 1 or 2 coats of gel stain.
Finishing Tip by Bob Flexner: Grain Reversal in Stained Pine
Pine is often stained to make it resemble a higher quality wood such as walnut, cherry or mahogany. You need to be aware, however, that the staining reverses the grain color, making what was the lighter-colored grain now the darker-colored grain. This happens because the softer, lighter-colored grain absorbs more of the stain than the much harder and denser darker-colored grain.
An example of this is shown in the first accompanying picture.The left side has been finished with a clear finish. The right side has been stained and finished.
The only way to keep this from happening is to spray the stain and not wipe off the excess. The problem is that this is similar to painting and obscures the figure of the pine.
An example is shown in the second picture where the same stain was applied and wiped off theleft side and sprayed and not wiped off the right side.
Spraying and not wiping off solves the problem of grain reversal (and also blotching), but it produces an entirely different look.
Finishing Tip by Bob Flexner: The Difference Between “Drying” and “Curing”
When a finish changes from a liquid to a solid film, it’s called “drying” or “curing.” Although these terms are often used interchangeably, they refer to different methods of forming the film, and understanding this difference helps in understanding finishes.
Drying refers to the evaporation of the solvent, which results in a solid film. Shellac and lacquer are the most common finishes that change to a solid by drying. (Liquid and paste waxes also work this way.) Finishes that dry entirely by solvent evaporation can be redissolved by wetting the surface of the finish film with the thinner—alcohol for shellac or lacquer thinner for lacquer.
Curing refers to a chemical reaction that occurs in the finish to bring about the change from liquid to solid. Varnish (including polyurethane varnish) and all the two-part catalyzed finishes, including even two-part water-based finishes, cure this way. Though there might be an initial evaporation of the thinning liquid, once the chemical reaction has taken place, the finish can’t be redissolved with that thinner—mineral spirits, lacquer thinner or water.
Common one-part water-based finishes change from a liquid to a solid using both drying and curing. The individual particles in the finish cure, but they stick together to form a film when all the liquid (water and a co-solvent, which is usually a glycol ether) evaporates. Rewetting the film with water won’t dissolve it, but rewetting with the co-solvent will separate the particles causing a partial redissolving.
Finishing Tip by Bob Flexner: Dealing with Grain Raising
Whenever water or any stain or finish that contains water comes in contact with wood, it causes the wood fibers to swell, which is called “grain raising” or “raised grain.” After the water has dried the wood feels rough to the touch, and thinly applied finishes also feel rough.
Raised grain occurs no matter how fine you sand the wood before wetting it. Because you can’t prevent raised grain if you use a water-based product, you need to deal with it so the final finish comes out smooth. There are two methods:
The first is to raise the grain and sand it smooth before applying the water-based product.
This is called raising the grain, sponging, whiskering or dewhiskering. Once sanded smooth, the grain won’t raise again nearly as much as it did with the first wetting.
After sanding the wood to about 150- or 180-grit, wet it with a sponge or cloth just short of puddling. Let the wood dry. Overnight is best, but three or four hours is usually sufficient if the air is warm and dry. Then sand the raised grain smooth with the same grit sandpaper you used last or one-numbered grit finer.
The goal is to smooth the raised grain without sanding deeper than necessary, in which case the newly exposed grain may raise again when wetted. Dull sandpaper works best because it doesn’t cut deeply so easily.
Raised grain is difficult to show in a picture, but it generally appears duller, as on the right side of the accompanying photo, which was wetted, dried overnight and not sanded smooth.
The second method is to “bury” the raised grain. If you’re applying a water-based finish, simply sand the first coat smooth after it dries, just as you would do with a solvent-based finish, except with a coarser sandpaper grit—for example, 220 grit. If you’re applying a water-based stain under a water-based finish, wait until after the first coat of finish has been applied and has dried to sand smooth. Sand lightly so you don’t sand through, which might remove some of the color.
Finishing Tip by Bob Flexner: “Equalizing” Sapwood
To achieve an even coloring with darker colored woods, it’s always best to use only heartwood to begin with. But this isn’t always possible. So you may want to “equalize” the coloring of the sapwood and heartwood.
One method is to bleach the wood using two-part bleach (sodium hydroxide and hydrogen peroxide). This will remove the coloring from the heartwood, so you can then stain the wood back to the color you want.
But it’s usually better to equalize the sapwood to the color of the heartwood.
The easier way to do this, if you intend to stain the wood dark, is to use a dark dye stain as shownin theaccompanying picture. Light colored dye stains don’t work as well,manned stains that contain binders, such as oil, water-based and lacquer stains, work even less well.
Though more difficult, the best way to equalize the coloring is to spray a dye stain just on the sapwood, asshown on the middle stripe in the second accompanying picture. You can then follow with a binder stain if you want to further equalize the coloring, as shown on the stripe second from the right. Finally, apply the finish, which will produce the true coloring.
Finishing Tip by Bob Flexner: Successfully Spraying Varnish or Oil Paint
The usual finishes that are sprayed are lacquer, shellac and water-based finish. These finishes dry fast, sometimes too fast in warm temperatures to successfully brush onto large surfaces. Spraying overcomes this problem.
Oil-based varnish, including polyurethane varnish and oil paint, can also be sprayed, of course, because any thin liquid can be sprayed. But you need to be aware of a significant difference. Varnish dries much slower than the other finishes, and unlike lacquer, shellac and water-based finish, each coat should be allowed to fully dry before the next coat is sprayed.
You may typically spray lacquer, for example, spray several coats at a time without allowing drying in between. The drying will be a little slower because there is more solvent in the thicker application that has to evaporate for the finish to harden. But the time difference won’t be great, and there won’t be any problems unless you build the finish so rapidly that it sags or puddles.
Varnish and polyurethane varnish (and oil paint) are different. For varnish to cure, oxygen has to work its way into the finish to cause it to crosslink. If you spray several coats of varnish one after another, the top surface will dry well before the varnish at the bottom, and this will significantly slow the penetration of the oxygen. The result will be a finish that feels dry at the surface but is still so soft that you can indent it with your fingernail, and it will remain like this for many days or weeks.
You can always allow way more time for the finish to cure all the way through. You can also speed up the curing by raising the heat in the room or applying heat with a heat lamp.
But this could cause the finish at the surface to wrinkle as the finish at the bottom dries and shrinks.
So it’s best to allow each sprayed coat of varnish or oil paint to fully dry (enough so sanding produces dust) before applying the next coat.
Finishing Tip by Bob Flexner: Finish Color Differences
Finishes differ in the amount of color they add to wood. Though you may not notice much of a difference if you are applying the finish over a stain, there is a significant difference when no stain or other coloring steps are used.
In the accompanying picture, you can see the differences clearly.
On the far left is paste wax. It adds almost no coloring to the walnut. Next is water-based finish, which also doesn’t add color, but it does darken the wood a little because of the penetration.
In the middle is nitrocellulose lacquer, which adds a slight yellowing to the wood. Clear (bleached) shellac is very similar. Next is polyurethane varnish. Varnishes can differ quite significantly in the amount of yellow/orange color they add to the wood. But most polyurethane varnishes are very close to this.
Finally, on the far right is orange (amber) shellac, which adds more orange coloring than any other finish.
So a big consideration when choosing a finish for unstained wood is the color you want to add. One of the main reasons, in fact, for choosing a water-based finish is to keep the color of the wood the same while still achieving a protective coat (which wax doesn’t provide).