CLOSED-PORE FINISHING
FILLING OPEN GRAINED WOODS
By Erick Kasner, PhD
Woods such as mahogany, rosewood, walnut, chestnut, oak and ash, among others have large pores in their surfaces. As such, they are referred to as open-grained woods. To achieve a smooth-as-glass-surface, though not necessarily glossy, the grain’s cavities must be filled and become flushed even with the ridges of the grain. That is particularly warranted on mahogany, walnut and rosewood; rarely done on oak, ash or chestnut. Otherwise, when top coating open grained woods the finish will “drop” into the cavities of the grain through gravity and/or grain absorption resulting in the finish having that “hungry look” which may not be appealing to the customer.
Closed pore finish filling is a matter of preference to the finisher or refinisher, as is style or, as dictated by the client. Many customers would prefer a natural open-pored look where, the texture of the wood shows through, particularly with oak, ash or chestnut. It is quite attractive in rubbed-effect satin or a matte finish. For a glossy but more so high gloss, smooth-as-glass finished wood surface, most woods should be grain filled, including oak and ash, so that no grain should be felt when running your hand across that surface. With woods having a small, or tight grain, such as cherry, birch and maple, grain filling can be an easier task. On the more porous woods, and specifically on oak and ash, and over a dark stain on mahogany or walnut, grain filling becomes a more difficult and a time consuming task – but does not have to be!
This article deals with pore or grain filling (used hereafter interchangeably), as a function of the type of wood being filled, and the reasons why to use or not to use a particular method. But, equally important, what can you do to increase productivity without detrimental consequences down the road, such as, crazing, cracking or dropping to the finish. It should be prefaced however, with the fact that, as of this writing, no “miracle” pore/grain filler exist on the market that will magically filled the grain and, is universally suitable across all grain filling demands. Equally significant, each method has its advantages and disadvantages, where the operator must exploit its pluses while minimizing its minus to best suit the needs.
There are four popular methods for pore filling: use of high-solids sanding sealer as grain filler; use of a high-solids topcoat; a lacquer-based quick-drying grain filler, and conventional paste wood filler. Each method of pore filling is quite functional, though constrained by productivity, type of wood grain being filled, the end result sought in terms of desired look and feel, and revenue to be received for the finished job. That is, properly pore filling with conventional paste wood filler yields the best results - smoothest finish, with best clarity and depth, and minimum amount of shrinkage. However, filling pores with a conventional paste wood filler can be time consuming and laborious, as such least productive of the four. Though can be very time consuming and hard to sand when used and applied improperly, we will show you herein how to better, easier and more productively grain fill with conventional (oil based) grain filler and still get that Steinway Piano look typically obtained through excessive work.
Table 1 at the end of the article summarizes each method to grain filling as discussed below, when and how to use, and the pros and cons of each.
Aside from the above, there are other methods used to grain fill, as for example, using a high solids polyester primer or two part automotive polyurethane, just to mention a few. These products, aside from being very expensive, are difficult to use and apply, have a relatively tight window of coat over or other constraints, thus only very experienced finisher or refinisher would use such products. Therefore, these methods are left out from this article and discussed in a follow up article. The follow up article will discuss the use of a conversion varnish (post catalyzed finish) as pore-filler.
For best grain filling results, it is recommended to sand the raw wood to 220 grit, finer if possible, prior to grain filling. The more you invest in up front sanding the less work you will have to do later on.
GRAIN FILLING WITH A SANDING SEALER
With small pore or tight grain woods such as birch, maple and cherry, and even on more porous woods as mahogany and walnut, high-solids, heavy-bodied, stearated sanding sealer can be used effectively to fill the grain. A heavy-bodied sanding sealer is one that is ready-to-spray as supplied, packing solids between 24 and 30%, by weight not volume. If the sanding sealer has to be thinned to spray, or solids are less than 24% when applied, grain filling results will be less than satisfactory due to excessive shrinkage.
However, this method is only applicable when top coating with a conventional lacquer or a pre-catatalyzed lacquer that is applicable over a stearated sanding sealer. Most pre-cat lacquers currently on the market and definitely post-cat lacquers, varnishes (conversion) and polyurethanes (two component) cannot be applied directly over stearated sanding sealers unless bridge-coated with a non-stearated vinyl sealer. The “metallic” soap found in most sealers and some vinyl sealers, typically as zinc or aluminum stearate, designed to make sanding easier, can and will interfere with proper cure of these topcoats. The acid catalyst built-in or post added to these top coats for cross linking purposes to increase hardness, mar and chemical resistance, will migrate to and react preferentially with the metal portion of the lubricate (stearate) in the sealer rather than cross-link within itself. That is, the acid of the catalyst is a stronger acid than stearic acid, thus the metallic portion from the “metallic soap will debond from the lubricating soap and bond with the stronger acid of the catalyst. The end result may be a soft finish with inferior durability properties. The “metallic soap” in sanding sealers, though not a pigment in the classical sense, can offer decent grain filling properties if applied properly.
When grain filling with a sanding sealer over open grain woods, apply a heavy double-pass coat with and against the grain. A double-pass coat constitutes first spraying with the grain, waiting 30 seconds or so then spraying against the grain or, in perpendicular direction from the initial coat. It does not matter whether the initial pass coat is with or against the grain, so long as the two pass coats are perpendicular to each other and closely as close equal in thickness. The purpose of spraying with and against the grain is to have the two 90 degree films flow into each other, providing better film characteristic and filling properties, particularly on porous woods. Allow the double pass coat dry at least two hours before applying the second double-pass coat. Do not sand the first double-pass coat. Apply a second double-pass coat in similar manner to the first double-pass coat. Let the second double-pass coat dry overnight - the longer the better. The extended drying time for both double-pass coats allows the latent solvents (tail solvents) to evaporate from the four films applied hence, providing better sanding properties and firm packing of the grain.
After the extended drying time, sand both coats down as far as possible without disturbing the stain. If filling natural mahogany, for example, sand the sealer coats all the way down to the wood grain. Use a 220 grit no-load sandpaper. This will pack the grain nicely and yield very good results. Machine sanding will pack the grain best, but riskier to burn through to the stain, if such exists. Unless you are filling oak or ash, two double pass coats of a heavy-bodied, high-solids sanding sealer will generally suffice to give you good grain filling results. On cherry, a single double-pass coat should suffice; two double-pass coats, however, would yield far better results.
If filling oak or ash, a third double pass coat may be necessary after the aforementioned sanding then sanded back down to the grain or as far as possible without disturbing the stain. Use 220 grit no-load paper or finer for this sanding. Giving this third coat at least two hours drying time prior to sanding will allow for some shrinkage and make sanding easier and pack the grain more effectively. Letting it dry overnight prior to sanding will yield even better results because of shrinkage and drop down.
Topcoat as normal with the finish and sheen of choice. It is not necessary to sand between the first two coats of the topcoat. However, sanding after the second coat is a must to remove imperfections, which will telegraph as added coats are applied.
Building a finish with sanding sealer rather than leaving its residue in the grain only must be avoided. First, to make them sand easier sanding sealers are soft by design, which will leave a soft base to the overall finish thus, susceptible to press marks and inferior mar resistance. Second, have lousy water, heat and alcohol resistance. Equally important, unless you are looking for that aged look in your grained filled finish, cracking of the finish within a year could also result. To achieve a crackled finish multiple coats of a soft sanding sealer is used (without sanding between coats) as an undercoat over an extremely hard topcoat. Moreover, that currently nicely filled finish will look much less attractive in about thirty days due to film shrinkage and dropping of the built up sealer.
When pore filling with a sanding sealer under pre-cat or post-cat lacquers or conversion varnishes, seal the sanded sealer with one to two coats (single pass) of a non-stearated vinyl sealer without sanding between the coats and after. Vinyl sealers are hard to sand but, more important, are highly flexible and offer excellent resistance to heat and moisture. Used under non-self-sealing pre-cat and post-cat finishes, nearly guarantees these high performance topcoats from cracking and chipping. As such, two coats are recommended and, be left in tact and not sanded. Any added staining or stain repair due to burn through should be done prior to applying the vinyl sealer coat(s).
GRAIN FILLING WITH A TOP COAT
Grain filling small pore woods such as birch, maple and cherry can also be achieved with high-solids, high-build top coat or a self-sealing, pre-cat lacquer that is ready to spray apply as supplied at minimum of 28% solids. If using a pre-cat or post cat lacquer to grain fill and finish, it is advisable to apply a double pass coat of non-stearated vinyl sealer first under most pre-cat and post-cat lacquers to avoid cracking as wood expands and contracts. Avoid using a catalyzed vinyl sealer so that it can move with the wood. Do not sand the vinyl sealer after its application. Let dry thoroughly then apply a double pass coat of the pre-cat or post cat lacquer wait at least two hours then sand down as far as possible, or at least smooth.
Over dark woods or dark stain will require many coats, a gloss version of the topcoat should be used up to the final coat, which will then dictate the final sheen. That is, if dull satin is the desirable final sheen, build with high gloss or at least gloss then, apply the final coat as dull satin. This will avoid interference to clarity typically caused by flatting agent in the finish.
For best results apply three double pass coats without sanding between the coats with ample drying time between each coat. No more than three coats per day and, at least two hours drying time between each coat. Let the three double pass coats dry overnight, or longer if possible because of shrinkage, then machine sand the finish down as far as possible without disturbing the stain (if exists) using 220 no-load sandpaper. As aforementioned, this will pack the grain and fill the pores. .
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Upon sanding down as far as possible to the wood grain without disturbing the stain, apply additional two (2) to six (6) double pass coats, lightly scuffing after every other coat. Sand the last to the final coat smooth prior to applying the final coat(s). Keep in mind that maximum thickness for most pre-cat and post cat lacquers is 5 to 6 wet mils, hence building up with too many coats could be prohibitive due to cracking. A very popular pre-cat lacquer currently on the market has a restriction to four wet mils coats or about 1.2 dry mils. Therefore, such restrictive pre-cat lacquers should not be used for grain filling.
This method can yield good results on mahogany, rosewood or walnut, but not so on oak, ash or chestnut unless “hot lacquer” are used to fill the grain and as a top coat. Within 60 days, the finish will shrink and the pores will show up again. A “hot lacquer” is a high solids (reaching 40% by weight), thick lacquer whose viscosity is reduced by first passing it through a heat exchanger at about 180 degrees just prior to spraying instead of reducing with a thinner. As such “hot lacquers” tend to shrink less than conventionally sprayed lacquers or sealers. However, development of high-solids-self-sealing-ready-to-spray pre cat lacquers and lacquer-based pore-fillers have made “hot lacquers” a finish of the past.
LACQUER-BASED QUICK-DRYING GRAIN FILLER
On medium pore woods, light grain or light stained larger pore woods, grain filling can be achieved successfully and rapidly with a lacquer-based wood grain filler. It contains a proprietary polyolefin based filler dispersed in lacquer rather than pigments, stearates or silicas. It can fill the grain rapidly, due to its quick drying time. However, clarity can be an issue if too many coats are applied and/or insufficient drying time is allowed between the coats. For best appearance on mahogany, rosewood walnut, only two double pass coats should be applied, with at least two hours drying time between coats. On natural oak, ash or chestnut, up to four (4) coats can be applied successfully with negligible sacrifice in clarity.