All levels of finishing are burdened with myths, but the types of finishes used by amateurs and sold in home centers and woodworking stores suffer the most. Myths about polyurethane are a good example of the problem.
What is Polyurethane?
Oil-based polyurethane is simply a type of varnish. It’s common alkyd varnish made with some polyurethane resin added. Alkyd is the resin used in almost all varnishes and oil-based paints. The polyurethane resin adds scratch, heat, solvent and water resistance to the alkyd varnish.
Pure polyurethanes (with no alkyd resin) are always two-part products. They cure in several ways: With the addition of moisture (an example is Gorilla Glue), with heat (many common plastics), or they are packaged as two separate components that cure after they are mixed (similar to the way two-component epoxy adhesives work).
The two-component polyurethanes are becoming more common in the furniture industry because they perform well and have a very high solids content, meaning less solvent to escape into the atmosphere.
One-component, “uralkyd” polyurethane has become so dominant in the woodworking and home-consumer world that it’s now becoming somewhat difficult to even find old-fashioned alkyd varnish.
Confusion has been added in the last decade or so with the introduction of water-based finishes, some of which combine polyurethane with acrylic resins. These finishes are sometimes labeled “polyurethane,” with no obvious reference to their being an entirely different class of finish, one that performs less well than oil-based polyurethane and has very different application characteristics.
This isn’t to say you shouldn’t use water-based polyurethane. Just be aware that it is an entirely different finish – a water-based finish. This article deals solely with oil-based polyurethane.
Myths are much more prevalent in finishing than in woodworking because finishes are chemistry, and you can’t always “see” differences in chemistry. For example, polyurethane and lacquer look the same, both in a can and on the wood, even though they have very different characteristics.
In contrast, woodworking is physics. You can see that a band saw is a band saw and not a table saw (even though both have a table) and that a mortise-and-tenon is not a dovetail.
So authors and manufacturers have much more opportunity to provide inaccurate information, intentionally or not, about finishes than about woodworking tools and procedures. And consumers are more vulnerable to misinformation – that is, “myths” – about finishing than about woodworking.
Once a myth gets into print, it’s common for it to be repeated endlessly until it becomes “fact,” simply because everyone says it. Here are some of the most common myths concerning polyurethane (and varnishes in general).
MYTH #1: Brush across the grain first to work the finish into the wood. All finishes soak perfectly adequately into the wood no matter how they are applied. They do this by capillary action, the same physical phenomenon that allows water and nutrients to rise from the ground to the top of a tree.
If it were necessary to brush across the grain first, or diagonal to it (as I’ve also seen advocated) to get the finish to penetrate into the wood, how would a sprayed finish penetrate?
The only benefit gained by brushing first across the grain and then with the grain (to line up the brush strokes with the grain) is to make the thickness of the application more even. But I never have a problem with some areas being noticeably thicker than others anyway.
More important, the longer polyurethane is brushed the more thinner evaporates, and this causes the finish to thicken and brush marks to be more pronounced.
MYTH #2: Thin the first coat 50 percent to get a good bond. This is an old myth that probably got its start because of poor understanding of the role of primers used under paint, and sanding sealers sometimes used under varnish and lacquer.
Primers do create a better bond for paint because they contain a higher ratio of binder (finish) to pigment. But finishes are all binder, so they bond perfectly well without a separate product.
Sanding sealers contain a soap-like lubricant that makes the sanding of the first coat easier and faster, so they are especially useful in production situations. But they weaken the bond of the finish, so unless you’re doing production work, you’re better off not using a sanding sealer.
Polyurethane bonds especially poorly to sanding sealers, so most manufacturers of polyurethane discourage their use. Furthermore, polyurethane sands easily, so there isn’t any need for a sanding sealer.
Nevertheless, the existence of primers and sanding sealers (and the recent addition of “SealCoat,” a dewaxed shellac from Zinsser, marketed for use as a “sealer” under polyurethane) have created a belief among some that something has to be put under the polyurethane to make it bond better.
And those who believe this have come up with the idea of thinning the finish.
In fact, the only benefit gained by thinning is faster drying. The thinner the layer of any finish, the faster it dries, and the sooner it can be sanded and the next coat applied. So there is a benefit, but it’s not improved bonding.
MYTH #3: Never shake the can or you’ll introduce bubbles. A corollary is: Never wipe the bristles over the rim of the can because this will also introduce bubbles into the finish.
This is a very old myth that is more “misleading” than “myth” because it’s true but it doesn’t matter. I don’t know when the myth started, but it is so well established that it’s even highlighted in bold letters on the cans of common brands such as Minwax and Olympic polyurethane.
Sure, if you shake the can, bubbles appear in the finish. And if you then brush the finish, bubbles appear in it. But they appear in the brushed finish even if you don’t shake the can. This should be the clue that shaking isn’t the critical factor – brushing is.
If the great majority of bubbles are the result of the turbulence caused by the rapid movement of the brush, the way to avoid bubbles could be to brush very slowly to reduce the turbulence.
The published instruction from one author who suggests doing this is to brush one foot every eight seconds and not brush back over. Try it. It’s almost impossible to brush this slowly. And not brushing back over to stretch out the finish and thin it on the wood – and also line up brush strokes – leads inevitably to runs on vertical surfaces and build-up on horizontal surfaces wherever you set down a finish-loaded brush.
So you’re going to get bubbles if you brush, whether or not you shake the can. The trick is to know how to keep the bubbles from drying in the finish.
Usually, bubbles pop out on their own. But on hot days when the skinning-over occurs faster and traps the bubbles, you may need to help the popping along by “tipping-off” using your brush. Brush lightly back over the finish right after application. (You should do this anyway to line up the brush strokes with the grain.) Most of the bubbles will disappear.
If the tipping-off doesn’t work well enough, add 5 or 10 percent mineral spirits to keep the finish “open” longer and give the bubbles more time to pop out.
Some brands of polyurethane, such as Minwax, bubble noticeably more than others, but this is rarely a problem because the bubbles tend to pop out quickly.
There are two ways to deal with bubbles that won’t pop out. The first is to wipe off the bubbled coat of finish using a rag dampened with mineral spirits, naphtha or turpentine. You can do this for 15 to 30 minutes after application without a problem. You won’t damage the coat underneath, and you don’t have to get all the polyurethane removed. Simply wipe until what is left is smooth and bubble-free.
The second method is to let the bubbled finish dry and sand it level before applying another coat. Thin this coat enough with mineral spirits so the bubbles have time to pop out.
The disservice caused by attributing bubbles to shaking rather than to brushing is that users become frustrated and lose confidence when they follow directions and still don’t avoid the problem.
MYTH #4: Thinning with naphtha makes polyurethane dry faster. Naphtha evaporates much faster than mineral spirits or turpentine. So the logic behind this myth, which is fairly new, is that the finish will dry faster if the thinner evaporates faster.
But, like all varnishes, polyurethane dries in two steps. The first is evaporation of the thinner. The second (and much longer) step is the curing, which is 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.
In fact, adding naphtha probably has no noticeable effect on the drying of the finish beyond the impact of thinning described at the end of the second myth.
MYTH #5: Thinning with Penetrol reduces brush marks. Painters have added Penetrol, a widely available additive, to oil paint for decades to reduce drag and brush marking, especially when painting in hot or cold weather, or in sunlight. But until recently I had never seen Penetrol recommended for use in polyurethane, which is usually applied indoors in more ambient working conditions and brushes easily without drag.
Penetrol is a slow-drying oil product that lengthens the tacky stage of polyurethane and oil paint. This creates more time for dust to settle and stick to the surface. Painters don’t mind because dust isn’t a big problem for them, but it is for furniture finishers.
Most woodworkers want their polyurethane to reach a dust-free stage faster, not slower, as evidenced by myth #4. What little benefit might be gained in reducing brush marking is more than cancelled out by increased dust nibs. (If you want to reduce dust nibs, thin the last coat of polyurethane by 25 to 50 percent so it dries faster.)
Additionally, because Penetrol is an oil, it can’t help but weaken the durability of polyurethane – though not enough to cause a problem in most situations, so far as I can tell. (See the next myth for the way to eliminate brush marks.)
MYTH #6: Slant the panel to reduce brush marks. I read this myth for the first time only recently. The idea is to get brush marks to flow together by tilting flat panels such as tabletops 5° or 10° off the horizontal.
Not only is this a tricky procedure that will lead to sagging if you aren’t careful to keep the finish thin on the surface (similar to the difficulty brushing vertical surfaces), but the procedure doesn’t make any sense. It’s not gravity causing one brush-mark ridge to sag into another that reduces brush marking. It’s gravity evening out the difference between the ridges and the troughs that eliminates brush marking.
Some brands of polyurethane level naturally better than others because of their formulation. But all polyurethanes can be made to level perfectly by adding mineral spirits. So the way to get a brush-mark-free finish is to sand the next-to-last coat level, then thin the last coat enough so it levels well.
The amount of thinner necessary varies with brands. My suggestion is to begin with about 25 percent mineral spirits and adjust from there.
MYTH #7: Scuff sand between coats to get a good bond. The purpose of this instruction is to create scratches in the surface so the next coat of finish can establish a “mechanical” bond. The finish “keys” or “locks” into the sanding scratches.
This myth is somewhat complicated. The first clue that sanding between coats isn’t so critical is that you rarely create scratches everywhere anyway. There are almost always gaps in your sanding – for example, in the pores, in recesses and often just because you aren’t being thorough enough.
And yet, the next coat usually bonds well anyway, especially if not a lot of time has gone by between coats. How often have you seen coats of polyurethane separating?
Here’s the way to approach sanding between coats: Do it anyway. Polyurethane dries slowly, so there are always dust nibs that should be sanded out before the next coat is applied.
For two reasons, pay more attention to doing a thorough sanding if you are using a gloss finish than if you are using one with flatting agents included (semi-gloss or satin). First, even tiny flaws show in gloss finishes, while they are often disguised in semi-gloss and satin. Second, finishes don’t “wet” and bond as well to gloss surfaces as they do to dull surfaces. (The dullness in semi-gloss and satin finishes is caused by a microscopically rough surface created by the flatting agent.)
Use a sandpaper grit that removes the flaws efficiently without creating deeper scratches than necessary. I almost always sand with #320 or #400 grit, regular or “P” grade. There have to be big flaws in the surface to require sanding with coarser grits.
I never back the sandpaper with a flat block when sanding between coats to remove dust nibs, though it would be all right to do this if the surface is truly flat. The biggest problem using a flat block is that you are more likely to clog the sandpaper and the clogs, or “corns,” will put deeper scratches into the finish that might telegraph through the next coat.
You can also abrade with steel wool or a synthetic abrasive pad, but neither cuts and levels as well as sandpaper.
Brushing polyurethane is not complicated, but somehow authors and manufacturers have succeeded in making it seem so. They have done this by introducing myths into their instructions. Some of the myths make no sense but do no harm. Others purport to solve a problem, and when following them doesn’t help, frustration sets in.
It’s unfortunate, but once a myth gets into print, nothing, not even an article such as this, seems to have any impact slowing its spread.
Probably the opposite, in fact. Conspiracy theorists know that merely repeating an “untruth” in order to debunk it tends more to solidify it in the minds of believers. PW
Bob is the author of “Flexner on Finishing.” Buy it in our store and get all the best finishing tips for your projects!