In the photo above we definitely see a very severe case. This is a pour on epoxy finish over copper. While finishing over copper is a separate challenge unto itself, the extreme cracking of the finish was due to being subjected to freezing temperatures and can happen with any finish. In this case the table was transported from Florida to Maryland in January and the table sat on a truck for several days.I would be the first to agree the copper (metal) was a contributing factor as the copper contracting due to the cold didn’t help things out at all.
I have however seen this several times before.A finish, if left in extreme cold, can and will crack. I have also observed that heavier film finishes are more subject to cracking. In all the cases I have seen , it occurred via a move from a warm climate to a cold one and the finishes were subjected to a rapid temperature change due to transport.
In the case of the copper table, the owners assumed it had been dropped, but there is no evidence of that, it simply froze and cracked, again the copper had a lot to do with it because the epoxy is simply lifting off which means that the adhesion was totally lost.
Be aware that it can happen. I have also see furniture stored in very cold conditions with no effect, thus it really seems to be the rapid change in temperature is a major contributor. If you are moving, be aware.
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When sanding wood in preparation for a stain or finish, you need to remove all the problems in the wood – mill marks, tear outs, gouges, etc. – with the coarsest grit sandpaper you’re using before moving on to finer grits (to remove the coarse-grit scratches). This means that the coarse-grit sandpaper you begin with should be able to remove the problems quickly and efficiently to reduce the amount of work required.
As an example, 100- or 120-grit sandpaper is usually coarse enough to begin with. Beginning with 150-grit sandpaper is usually inefficient because you have to sand too long to remove the problems.
On the other hand, with factory pre-sanded veneered plywood or mdf, beginning with 150-grit sandpaper is usually adequate.
These are general observations. We all sand differently, so you may want to adjust the beginning grit you sand with depending on the problems you want to remove. But the basic rule still applies: It’s most efficient to remove all the problems with the coarsest grit you’re using before moving to finer grits.
Testing Finishes for Heat Resistance
Resistance to damage from hot objects is an important finish quality for tabletops and counter surfaces in kitchen and dining areas. The type of finish you’re using will be a strong clue to its resistance. For example, oil-based polyurethane and catalyzed lacquer are very resistant to heat damage, while shellac, lacquer and water-based finishes are less so.
There are two tests for heat resistance that are easy to do.
To test for dry heat resistance, place a metal cup or pot containing water heated to just below boiling on a fully cured finish, as shown in the accompanying picture. Remove the cooled container after an hour and look for splits, indentations or discoloration in the finish.
To test for wet heat resistance, do the same as for dry heat resistance, but place a cotton cloth or cheesecloth wetted with the same hot water under the cup or pot. After an hour check the surface for splits in the finish or discoloration.
Testing Finishes for Scratch Resistance
Finishes vary in hardness and scratch resistance. To a large extent the relative hardness of various finishes are known. For example, oil-based polyurethane is more scratch resistant than lacquer or shellac. So are catalyzed finishes and water-based finishes. But what if you want to be more exact about the differences, or want to compare brands within any single type of finish?
The easy way to do this is with architect’s pencils because these vary in hardness themselves.
Begin by buying a set of architect’s drawing pencils ranging in hardness from about 2B (soft) to 5H (hard). Sharpen each pencil using a knife so you leave the sharp cylindrical edge of the lead intact. If you damage this edge, or if it becomes worn, sand it flat, holding the pencil 90 degrees to the sandpaper.
Beginning with one of the softer pencils, hold it as you would for writing and push it forward across the fully cured finish as shown in the accompanying picture. Maintaining equal pressure, follow with pencils of increasing hardness until you find one that cuts into the finish.
The hardness rating for that finish is the number of the previous pencil – the hardest lead that doesn’t cut.
One of the most difficult aspects of wood preparation is knowing how much to sand to remove all the machine marks and other flaws. Here’s a trick that may make it easier.
Draw some pencil marks on the wood. Then sand the wood until you have removed them entirely.
Sanding is very personal. We all sand differently. We use different grits, more or less pressure, more or fewer passes and with more or less wear on the sandpaper. So you may determine that, for you, sanding off the pencil marks once is enough. Or you may determine that drawing a second set of marks after the first has been removed and then sanding off these marks works better. In other words, doing it twice.
You should never have runs or sags in your dried finish. The trick to avoiding them is to spot them before the finish dries and remove them with a brush. The way to see runs and sags developing is to look at the surface in a reflected light, as shown in the accompanying picture. You may need to move your head, walk around, or even arrange some special lights other than the overhead lights or the light coming in through a window.
Once you have spotted the problem, use a brush to brush out the excess finish even if you’re spraying. If there is too much finish to brush out successfully, lift some off the surface with the brush and deposit it onto another part of the object, wipe the brush dry with a cloth, or drag the brush over the lip of a jar or can to remove the excess.
In most cases when repairing a finish (for example, removing a water ring, touching up a scratch or ding, or doing a burn-in or epoxy fill to a gouge) it’s most important to get the sheen the same as the surrounding surface and make the surface feel smooth. Even if the color of the repair doesn’t match exactly, the repair will be successful because people will think the mismatch is a flaw in the wood.
If you don’t get the sheen right, however, people will notice the repair immediately when they see it in a reflected light. Even if the do get the sheen right but not the color exact, most people will run their fingers over the repair, so it’s also important to make it feel smooth.
The two widely available pigment colorants for oils and varnishes are oil colors and Japan colors. The difference is that oil colors are pigment ground in linseed oil while Japan colors are pigment ground in varnish.
So the difference in practice is that Japan colors dry faster and harder than oil colors, though if you were to mix an oil color with varnish, it should dry well.
The name “Japan” comes from the attempt in the West to imitate Japanned furniture (also called Japanese or Oriental Lacquer) that was imported in the 17th and 18th century. So the harder drying and glossier varnish base worked better than an oil base.
In my experience you can also use widely available universal colorants (normally meant for tinting latex paint) in oils and varnishes successfully if you let these colorants sit in the binder overnight while stirring occasionally.
There are two critical elements that make for a great finish – selecting the right brush, and using the right technique. Practice counts as well. You don’t expect to cut perfect dovetails the first time round, nor should you expect to achieve a perfect finish without practicing your finishing technique. I frequently choose brushing because I have a small shop and brush clean-up is fairly quick.
Most of my finishing is done with shellac or varnish. Occasionally I’ll use a water-based finish for light-coloured woods when I want a super clear finish. However, I’ve never used lacquer, so my comments won’t apply to this type of finish.
The most important part of a brush are its filaments (aka ‘bristles’ or ‘hairs’). It’s the filaments, along with your brushing technique, that will have the greatest impact on the quality of your finish. With proper care brushes will last for years, so save yourself a lot of frustration by using quality brushes right from the start, and keeping the brushes in top condition. Cheap brushes will invariably result in cheap looking finishes.
The shape of the brush tip is also important. Avoid brushes that are trimmed flush across the end. A chisel tip will give you much better performance.
Basically, there are three kinds of filaments used in brushes – natural (usually hog bristle, ox hair, or badger hair), synthetic (made from polyester or nylon), and natural/synthetic blends. The general consensus is to choose natural brushes for shellac and varnish, and synthetic for water-based. However I’ve had good success using natural/synthetic brushes and synthetic brushes with shellac and varnish when I thin the finish by 10% to 20% with its solvent.
Natural brushes work well on shellac and varnish because they carry more finish (i.e. to ‘load’ the brush), than synthetic brushes, so that you can lay on more finish with each brush stroke, and maintain a wet edge as you work. When buying a natural brush, look for filaments that have flagged ends – a split at the end of each individual filament. The flagged ends enable the brush to hold more finish and release it with minimal brush marks.
There are two general types of natural brushes. The most common are made from the hair of hogs, typically referred to as ‘bristle’, or ‘China bristle’. Any brush that’s labelled ‘bristle’ is going to be made of hog hair. Hog hairs have naturally split ends. They also taper from the base to the tip, which makes the hair strong yet gives it a lot of spring, so that it maintains its shape in use.
Higher quality natural brushes are made from ox or badger hair, which is a softer, finer, and more pliable filament than ox bristle. You pay a premium for ox or badger brushes, but they do leave a finish that is as good as it gets.
With any natural brush you’ll want to remove any dust or loose filaments before you first use the brush. Shake it out vigorously, then dip it about 1/2 of its length into the appropriate solvent for the finish you’ll be using, and then gently tap it against the side of the container.
For water-based finishes use synthetic brushes, because the water in the finish will cause natural filaments to swell and lose their stiffness. Most synthetic brushes have polyester or nylon filaments, or some combination of the two. Neither are as absorbent as natural filaments, which means you have to load up the brush more frequently. Most manufacturers whip the filaments to split the ends into flagged ends to give a smoother finish.
For large surfaces use a 2″ to 4″ wide brush, while for smaller panels, frames, edge work, and legs, choose a narrower 1″ to 1-1/2″ brush. It’s true that an angled (or ‘sash’) brush allows finer control in corners and tight spaces, but rather than adding yet another brush to my kit I simply switch to a narrower brush, which I find works just as well.
If you typically work with small items, then pick up a couple of small artist brushes (available with both natural and synthetic filaments). They excel at working in tight spaces, and the super fine filaments allow the finish to flow onto the work easily with virtually no visible marks.
When applying a finish think about scale. A large surface has the capacity to absorb finish at a faster rate than a smaller surface or narrow trim work. Using a heavier load on the larger surface makes it easier to maintain a wet edge. Conversely, applying too much finish to a carved or contoured piece will result in runs and sags that can be difficult to correct. With practice, and paying attention to the feel of the brush as you apply the finish, you’ll learn to identify the correct load for each situation.
When loading the brush you only need to dip the bottom third or half of the brush into the finish, and then wait for the finish to wick into the brush. Before moving the brush to the project, adjust the amount of the load by lightly pressing the brush against the side of the container.
Rather than brushing from one end of a panel to the other end, begin by placing the brush a couple of inches from the end of the panel and move the brush to one end. After completing that stroke, return to the starting point and brush back to the opposite end. As you get to either end of the panel the brush should just glide off the top without moving down the edge of the panel. After completing the first pass, go back and use the tip of the brush to gently work the finish across the edge.
Brush cleaning doesn’t have to be arduous or time consuming. If you’re applying a finish over a day or two, then don’t bother cleaning the brush. Instead, suspend it in a container of the solvent for the finish – just make sure the tip doesn’t stand on the base of the container or it will curl up. Before using again, lightly press the excess solvent out of the brush on the side of the container.
To clean a brush used for varnish or water-based, you want to rinse the brush several times in the solvent for the finish you’re using (mineral spirits for varnish, water for water-based). I do this three or four times, and then wash the brush several times with warm water and dish soap. For a varnish brush, rinse the brush first with a citrus-based cleaner and then again with water to remove the final traces of solvent from the brush.
Shake out any excess moisture by swinging it up and down several times, and then wrap the filaments with a piece of paper – I use strips from paper grocery bags. This allows the bristles to dry, and keeps dust from getting into the filaments.
My particular brush arsenal consists of two brushes for each type of finish. For shellac I use a 2″ wide ox-hair and a 3/4″ synthetic; for varnish, a 2″ and a 1″ bristle brush; and, for water-based finishes a 2″ synthetic and a 3/4″ synthetic. The type of woodworking you do will obviously affect your choice of brush.
Buy good brushes, practice applying the finishes you intend to use, take the time to clean the brushes after use, and you’re three quarters of the way to a perfect finish. The last quarter comes from experience.
So, you’ve just finished that stellar table top and you’d like to give it a ‘smooth as glass’ finish. What’s a woodworker to do? Why, ‘Fill and Finish’ of course.
There are two kinds of ‘fillers’ – putty type fillers used to fill scratches, dents, and holes in wood, and grain (aka pore) fillers that serve to level out the surface of open grained woods. It’s the latter filler that concerns us here.
Woods such as oak, ash, elm, mahogany, chestnut, walnut, wenge, and teak are characterized as having ‘open grain’ because the wood pores are large. In contrast, ‘closed grain’ woods like hard maple, cherry, poplar, beech, and bubinga, have much smaller pores.
You don’t have to fill the pores of any wood before applying a finish – and a lot of woodworkers don’t. It’s a matter of what you want the final surface to look (and feel) like. If you want to achieve a super smooth finish on open grained woods fairly quickly, then your best bet is to fill the pores before you apply your chosen finish. Good candidates for pore filling are large horizontal surfaces – the tops of tables, desks, sideboards, dressers, fancy boxes, and the like.
Sealing is Not Pore Filling
A sealer coat, washcoat, or a sanding sealer won’t necessarily fill wood pores.
The first coat of finish you apply to a wood surface effectively seals the wood, which is why it’s called a ‘sealer coat’. However this coat doesn’t necessarily fill up all the pores – especially on those open-grained woods I mentioned above.
When you apply that first coat of finish (i.e. the sealer coat), the wood fibres will likely swell, giving the surface a fuzzy texture (known as ‘raised grain’). This is especially apparent with water-based finishes. Before you apply subsequent coats of finish you’ll want to sand off these fibres. Once you’ve sanded back the raised grain, it won’t occur again.
To make it easier to sand back this first coat of finish, many woodworkers will thin the finish with it’s appropriate solvent, or use a 1 pound cut of shellac. This thinned version of the finish is called a ‘washcoat’. In production shops, where they often spray on lacquer or varnish, they’ll typically use a commercial product called a ‘sanding sealer’ rather than a washcoat.
Oil or Water
There are two basic types of grain fillers – solvent (or oil) based, and water-based. You’ll want to use an oil-based filler only with an oil-based finish. However, you can use a water-based grain filler under either an oil-base or water-based finish. I use water-based fillers all the time now. They don’t emit toxic fumes, making them safer to use, better for my health, and better for the environment, and they’re easy to apply and compatible with any finish. The only issue is that, as with all water-based products, they dry fairly quickly. The water-based filler I’ve recently been using is Aqua Coat’s Clear Grain Filler.
If you’ll be finishing with a penetrating finish (tung, linseed, wiping varnish, or oil/varnish blends such as Danish oil), it’s best not to use a pore filler, as these as they don’t cure hard enough.
Colour or Not
If you plan to stain the wood surface, you can do so before or after applying the filler. The fillers themselves come in clear, natural, and some colours. You can add a colorant to the clear or natural fillers. Regardless, the filler will likely take the stain differently from the surrounding wood, and sanding after you stain might result in some of the stain being removed. To avoid disappointment, it’s best to do some test staining on scrap wood cut-offs before you commit to your project.
Easy to Apply
The water-based filler that I currently use, Aqua Coat Clear Filler, is very easy to apply. The only caveat is that you have to work in small sections at a time, and reasonably fast. Though the process is quite simple, I’d suggest you begin by sealing and filling a test piece or two – particularly if you’ve never done this before.
I almost always sand with a random orbital sander, up to 180-grit, followed by hand sanding with 220 grit, in the direction of the grain. After wiping the surface clean with a tack cloth I apply a seal coat. Because I use a fair amount of shellac, and usually have a batch laying about, I seal the surface with a 1 pound cut of blond shellac. I let the shellac dry for a couple of hours and then lightly sand the surface with a piece of used 320- or 400-grit sandpaper. Just kiss the surface to remove any bits of raised grain and dust nubs.
You don’t need to mix Aqua Coat Clear Filler before using it. Just scoop a tablespoon or so and drop it on the work surface. The objective is to pack the filler into the pores of the wood. The easiest way I’ve found to do this is with a rubber squeegee. I suppose you could also use an old credit card. Work the filler diagonally to the grain and don’t forget to do the end grain. Aqua Coat spreads easily, and I find that I can do about one square foot of surface in under a minute. I’ve never felt the need to thin the filler with water.
Using a squeegee makes it easier to control the filler, and usually I end up with a very thin, even coat spread over the work area. You can easily see if all the pores are filled by looking at the surface at an angle, against a bright light beamed obliquely to the surface. Once the surface begins to dull I wipe it down with a cloth, across the grain, just enough to remove any bits of paste that haven’t gone into the pores. I’m not overly concerned about getting every single micrometer of space filled, as I always lay on a second coat of filler after the first coat has dried for at least an hour. Before repeating the process I’ll lightly sand the surface with 320- or 400-grit paper. While I like to let the surface cure for a few days before applying the finish, I don’t think it’s absolutely necessary.
To spray a uniform thickness on a wide surface such as a tabletop, hold the spray gun perpendicular to the work (lock your wrist so you don’t rock the gun back and forth) and overlap each spraying stroke by 50 percent.
Begin with the spray pattern half on and half off the front edge of the surface and then overlap each additional stroke by half until the last, which should be half on and half off the surface.
So you are actually applying a double thickness and calling it one “coat,” but it’s the best way to get an even build.
The thinner and clear-up solvent you should use with shellac is denatured alcohol. This is ethanol, the same alcohol that is included in beer, wine and liquors. But it is made poisonous so it can be sold without liquor taxes.
Sometimes you’ll see methanol (methyl alcohol) sold in paint stores. Methanol works fine for thinning shellac, but it is quite toxic if you are around it for a long time breathing the vapors. So it isn’t a good idea to use methanol unless you are working with a good exhaust.
Isopropyl rubbing alcohol is no good for thinning shellac because it contains about 30% water. The water will cause the shellac to turn white, or “blush.” If you have access to 95-to-100 percent pure propanol or isopropyl alcohol, you could use it for thinning without a problem.
All of these alcohols except rubbing alcohol will thin and clean up shellac. The difference among them is evaporation rate. Methanol evaporates the quickest. Denatured alcohol is next. And propanol and isopropyl alcohol are the slowest.
Another concept we drill in finish training is controlling the environment in which spraying occurs. It always amazes me how many finishers tell me that they prefer to spray outside, because it is just easier than creating clean, controlled conditions in the workshop.
While this may seem convenient to you as the technician, it is not at all appreciated by your spray gun, or the product being sprayed.
Aside from the obvious risks of contaminants and unpredictable lay down of product, the primary problem with outdoor spraying is the unpredictability of wind. The slightest random breeze outdoors will wreak havoc on the transfer efficiency of your gun. In other words, you are shooting at your target, and wind is passing through the surface area, moving and diffusing your sprayed fan.
Remember, HVLP spraying is low pressure. The sprayed fan is very fine, which is the benefit of HVLP spraying to begin with. Because of product loss in the air, outdoor spraying often requires 4-5 coats to achieve the level of finish that can easily be laid down in 2-3 coats in the controlled environment of the workshop.
The wasted product is a big deal, because fine finishing products are often pricy. Why spend twice as much on materials? Further, the time that it takes to create additional finishes due to loss of transfer efficiency puts projects behind schedule. Whether you are a hobbyist or a pro finisher, time is just too precious to waste.
So, with spring coming, rather than embracing the nice weather for outdoor spraying, open up the windows in the workshop and give it a good spring clean and organize. Set up a dedicated spray area that you can control, both in terms of cleanliness and air flow (a simple exhaust fan out a window). The best way to exhaust is to be drawing air away from the finish area, not randomly into it or through it.
These two simple adjustments – straining habits and controlled environment – will make a profound difference in your finished results, while saving you time and money. It’s all about quality and efficiency.
Happy spring and happy spraying!
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