How Much Thinner Should You Add To Polyurethane

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No reward. Information technology'due south not necessary to brush across the grain, as I'yard demonstrating here, to work the end into the wood. The terminate penetrates perfectly well past capillary action no matter how it is applied.

Oft-repeated 'rules' that are, quite just, wrong.

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 proficient example of the trouble.

What is Polyurethane?

Oil-based polyurethane is merely 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 e'er two-part products. They cure in several ways: With the addition of moisture (an instance is Gorilla Glue), with heat (many common plastics), or they are packaged as two separate components that cure after they are mixed (like to the fashion two-component epoxy adhesives work).

The 2-component polyurethanes are becoming more mutual in the furniture manufacture because they perform well and accept a very high solids content, pregnant less solvent to escape into the temper.

I-component, "uralkyd" polyurethane has become then dominant in the woodworking and domicile-consumer world that it'south now becoming somewhat difficult to fifty-fifty find old-fashioned alkyd varnish.

Confusion has been added in the last decade or so with the introduction of h2o-based finishes, some of which combine polyurethane with acrylic resins. These finishes are sometimes labeled "polyurethane," with no obvious reference to their beingness an entirely unlike course of finish, one that performs less well than oil-based polyurethane and has very unlike application characteristics.

This isn't to say y'all shouldn't use water-based polyurethane. Just be aware that it is an entirely dissimilar end – a water-based cease. This article deals solely with oil-based polyurethane.

The Myths

Myths are much more prevalent in finishing than in woodworking considering finishes are chemistry, and you lot can't ever "see" differences in chemistry. For example, polyurethane and lacquer look the same, both in a tin can and on the wood, fifty-fifty though they have very different characteristics.

In contrast, woodworking is physics. You lot can encounter that a band saw is a ring saw and not a table saw (even though both have a tabular array) and that a mortise-and-tenon is non a dovetail.

So authors and manufacturers accept much more opportunity to provide inaccurate data, 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 incessantly until it becomes "fact," merely because anybody says it. Hither are some of the most common myths concerning polyurethane (and varnishes in full general).

■ MYTH #ane: Castor beyond the grain first to piece of work the finish into the wood. All finishes soak perfectly adequately into the woods no matter how they are practical. They do this by capillary action, the same concrete phenomenon that allows h2o 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 information technology (equally I've besides seen advocated) to get the terminate to penetrate into the wood, how would a sprayed stop penetrate?

The only do good gained by brushing first across the grain and and so with the grain (to line upwardly the brush strokes with the grain) is to make the thickness of the application more than fifty-fifty. Simply I never have a trouble with some areas being noticeably thicker than others anyhow.

More important, the longer polyurethane is brushed the more thinner evaporates, and this causes the stop to thicken and castor marks to be more pronounced.

■ MYTH #two: Sparse the first coat 50 percent to become a skillful bail. This is an former myth that probably got its get-go considering of poor agreement of the role of primers used under paint, and sanding sealers sometimes used nether varnish and lacquer.

Sanding. Not only does polyurethane not need thinning or a special product to create a better bond to the woods, but it also sands hands on its own without clogging sandpaper.

Primers do create a better bail for paint considering they contain a higher ratio of binder (finish) to pigment. But finishes are all binder, so they bail perfectly well without a dissever product.

Sanding sealers contain a soap-like lubricant that makes the sanding of the beginning coat easier and faster, so they are peculiarly useful in product situations. Merely they weaken the bond of the terminate, so unless you're doing production work, you lot're improve off non using a sanding sealer.

Polyurethane bonds especially poorly to sanding sealers, so about manufacturers of polyurethane discourage their use. Furthermore, polyurethane sands easily, and so there isn't any need for a sanding sealer.

Still, the beingness of primers and sanding sealers (and the recent addition of "SealCoat," a dewaxed shellac from Zinsser, marketed for use as a "sealer" under polyurethane) accept created a conventionalities among some that something has to be put under the polyurethane to make it bond better.

And those who believe this have come with the idea of thinning the finish.

In fact, the only benefit gained by thinning is faster drying. The thinner the layer of whatsoever finish, the faster it dries, and the sooner it tin can be sanded and the adjacent coat applied. So at that place is a do good, merely it's not improved bonding.

■ MYTH #3: Never shake the tin or you'll innovate bubbles. A corollary is: Never wipe the bristles over the rim of the can because this will also introduce bubbles into the finish.

Bubbles. If yous shake a terminate vigorously, equally I did this jar containing polyurethane (above), thousands of tiny bubbling appear. These can be transferred to the forest past a castor. But bubbles appear on the forest even when you lot don't milkshake the finish because of the turbulence caused by the castor movement, as I'thou demonstrating hither with polyurethane I didn't shake (below). And so it'south more than important to know how to bargain with the inevitable bubbles than to follow instructions not to shake.

This is a very old myth that is more "misleading" than "myth" because it'due south truthful just it doesn't matter. I don't know when the myth started, merely it is and so well established that information technology'southward even highlighted in bold letters on the cans of common brands such as Minwax and Olympic polyurethane.

Sure, if you shake the tin can, bubbles appear in the cease. And if you then brush the stop, bubbles appear in it. Just they announced in the brushed cease even if you don't milkshake the tin can. This should be the clue that shaking isn't the disquisitional 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 exist to brush very slowly to reduce the turbulence.

The published instruction from one writer who suggests doing this is to brush one pes every eight seconds and not castor back over. Try it. It's nigh impossible to brush this slowly. And not brushing back over to stretch out the cease and sparse it on the wood – and too line upwards castor strokes – leads inevitably to runs on vertical surfaces and build-upwardly on horizontal surfaces wherever you prepare down a cease-loaded brush.

And so yous're going to go 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 castor. Brush lightly back over the end right after application. (You should do this anyhow to line upwardly the brush strokes with the grain.) Most of the bubbles will disappear.

If the tipping-off doesn't work well enough, add together 5 or ten percent mineral spirits to keep the finish "open up" longer and requite the bubbles more time to pop out.

Some brands of polyurethane, such as Minwax, bubble noticeably more others, but this is rarely a trouble because the bubbles tend to popular out chop-chop.

In that location are ii ways to bargain with bubbles that won't popular 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 subsequently application without a problem. You won't damage the coat underneath, and you don't take to go all the polyurethane removed. Simply wipe until what is left is shine and chimera-free.

The second method is to let the bubbled finish dry out and sand information technology level before applying another coat. Thin this coat plenty with mineral spirits so the bubbles have fourth dimension to pop out.

The disservice acquired by attributing bubbling to shaking rather than to brushing is that users become frustrated and lose conviction 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. And then the logic behind this myth, which is adequately new, is that the stop will dry faster if the thinner evaporates faster.

Gummy phase. Once the bubbles accept popped out, you lot want the polyurethane to dry as fast as possible to get beyond the pasty stage (demonstrated here by a fingerprint in the terminate) and then dust nibs are kept to a minimum. Adding faster-evaporating naphtha to the finish doesn't speed the drying of this stage beyond what is achieved but by the thinning. You could just as well use mineral spirits.

But, like all varnishes, polyurethane dries in two steps. The showtime is evaporation of the thinner. The second (and much longer) step is the curing, which is the crosslinking brought almost by the introduction of oxygen from the air.

When yous employ polyurethane, you detect that it stays wet on the surface for a curt time as the thinner evaporates. And so the finish goes into a tacky or viscid phase 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, calculation naphtha probably has no noticeable outcome on the drying of the finish beyond the impact of thinning described at the stop of the 2nd 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 castor marking, especially when painting in hot or cold weather, or in sunlight. Simply until recently I had never seen Penetrol recommended for apply in polyurethane, which is usually applied indoors in more ambient working conditions and brushes easily without elevate.

Penetrol. Adding Penetrol (a widely bachelor additive for oil paint) to polyurethane slows its drying and then dust has more than time to settle and stick to the cease. Any possible proceeds in reduced castor marking is more than cancelled out by the added dust nibs.

Penetrol is a slow-drying oil product that lengthens the tacky stage of polyurethane and oil paint. This creates more than fourth dimension for dust to settle and stick to the surface. Painters don't heed because dust isn't a big problem for them, but it is for furniture finishers.

Most woodworkers want their polyurethane to achieve a grit-free stage faster, not slower, as evidenced past myth #4. What niggling benefit might be gained in reducing brush mark is more than cancelled out past increased dust nibs. (If you lot want to reduce dust nibs, thin the last coat of polyurethane by 25 to l percent and then it dries faster.)

Additionally, because Penetrol is an oil, information technology tin can't help but weaken the immovability of polyurethane – though not plenty to cause a problem in most situations, so far every bit I can tell. (Run across the next myth for the style to eliminate brush marks.)

■ MYTH #vi: Slant the panel to reduce brush marks. I read this myth for the kickoff time only recently. The idea is to get brush marks to flow together by tilting apartment panels such equally tabletops five° or ten° off the horizontal.

Brush marks. The way to utilise a castor-marker-free end using polyurethane is to first sand out the brush marks on the next-to-last coat using a flat backing block and a mineral oil, mineral-spirits or water lubricant. So sparse the concluding coat about 25 per centum with mineral spirits so it levels well.

Not only is this a catchy procedure that will atomic number 82 to sagging if yous aren't conscientious to keep the finish thin on the surface (like to the difficulty brushing vertical surfaces), but the process doesn't make any sense. It's non gravity causing ane brush-marking ridge to sag into another that reduces brush marker. It's gravity evening out the difference between the ridges and the troughs that eliminates brush marker.

Some brands of polyurethane level naturally amend than others because of their formulation. But all polyurethanes tin can be made to level perfectly by adding mineral spirits. And so the way to get a brush-mark-costless finish is to sand the next-to-final coat level, so thin the final coat enough so it levels well.

The amount of thinner necessary varies with brands. My suggestion is to begin with most 25 per centum mineral spirits and conform from there.

■ MYTH #vii: Scuff sand between coats to get a good bond. The purpose of this teaching is to create scratches in the surface so the next coat of finish tin institute a "mechanical" bond. The end "keys" or "locks" into the sanding scratches.

Camber panel? Slanting a panel 5° or ten° is not the way to reduce brush marking, but this has actually been suggested recently. You would take thought furniture finishers working during the final several hundred years would take figured this out if information technology were true.

This myth is somewhat complicated. The first inkling that sanding betwixt coats isn't then critical is that you rarely create scratches everywhere anyhow. There are almost ever gaps in your sanding – for example, in the pores, in recesses and frequently just because you aren't being thorough plenty.

And yet, the next glaze usually bonds well anyway, especially if not a lot of time has gone by between coats. How oftentimes have you lot seen coats of polyurethane separating?

Hither's the way to approach sanding between coats: Do it anyhow. Polyurethane dries slowly, so there are always grit nibs that should be sanded out before the adjacent glaze 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 i with flatting agents included (semi-gloss or satin). Showtime, even tiny flaws prove in gloss finishes, while they are often disguised in semi-gloss and satin. 2d, finishes don't "wet" and bond as well to gloss surfaces every bit they do to tedious surfaces. (The dullness in semi-gloss and satin finishes is caused by a microscopically rough surface created past the flatting amanuensis.)

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" course. 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 information technology would be all correct to do this if the surface is truly flat. The biggest problem using a flat block is that y'all are more likely to clog the sandpaper and the clogs, or "corns," will put deeper scratches into the finish that might telegraph through the adjacent glaze.

Y'all can likewise abrade with steel wool or a synthetic annoying pad, but neither cuts and levels too as sandpaper.

Determination

Scuffing. The principal reason to sand polyurethane betwixt coats is to level grit nibs and other flaws, not to create a meliorate bond. This is demonstrated hither by the gloss remaining in the pores after sanding the surface. The next coat of cease notwithstanding bonds in the pores even though the stop there is rarely scuffed.

Brushing polyurethane is not complicated, but somehow authors and manufacturers take succeeded in making information technology seem so. They accept washed this by introducing myths into their instructions. Some of the myths brand no sense but do no damage. Others purport to solve a trouble, and when following them doesn't help, frustration sets in.

It'southward unfortunate, just once a myth gets into impress, naught, non even an article such as this, seems to take whatsoever bear on slowing its spread.

Probably the opposite, in fact. Conspiracy theorists know that only repeating an "untruth" in order to debunk it tends more to solidify it in the minds of believers.

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