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Applying Finishes to Plywood-Based Items

Perhaps no aspect of woodworking inspires as much trepidation as applying a finish to a completed project. The right finish will enhance the wood’s natural beauty and highlight your workmanship. Done correctly, it represents the crowning achievement of a job and can almost make a wooden object come alive. Done poorly, it highlights and magnifies construction imperfections while almost commanding one’s gaze to seek out those areas.

The finish of a project starts with the very first step; selecting the material. Material with defects, such as splits, knotholes and dents is going to be much harder to finish well, than finish that is free of damage and sanded smooth. This is especially true for plywood products, where the damage can easily go through the face veneer, making repairs difficult.

Nevertheless, plywood is a popular building material for a variety of furniture and DIY projects. Part of that is because plywood gives you a sheet of wood, large enough for tabletops and cabinet sides, without having to go through the trouble of laminating boards together and for a much lower price than buying all that wood to laminate together. Nevertheless, finishing plywood comes with many of the same finishing challenges as projects constructed of solid wood. The type of finish used for plywood depends on the project and the type of plywood used.

Plywood grades and projects

Part of selecting the material for your project is determining what grade of plywood you are going to use. The general term of “plywood” can refer to a wide range of products, including many which don’t fit the more traditional definition of plywood. Each has its purpose and you will likely find yourself working with a wide variety of them at one time or another.

Regardless of the type, plywood normally comes in 4’ x 8’ sheets with a face (top) side and a back (bottom) side. The face and back on traditional plywood are graded according to the quality of the wood and surface preparation. While grading is standardized, the grading system used for hardwood plywood products is different than it is for softwood plywood.

In the USA, the face of hardwood plywood is given a grade fromA–Dwith A being the highest quality and the back is graded from 1–4, with 1 being the highest grade. Therefore, A1 plywood is the highest quality (furniture quality) and comes naturally at the highest cost. In Europe, the only difference comes from the grading of the back side (given grades from B–D with B being the highest).

Softwood plywood uses the same letter grading system for both the face and back sides. In general, you won’t find A, B or AB grade softwood plywood, although it does exist. The problem is that most lumberyards don’t bother to carry it, as the demand is low. Typically, the only softwood plywood that they carry is construction grade, which will usually be CD grade. However, the major home improvement centers carry sanded BC grade softwood plywood.

For high-end furniture and cabinetry projects, using “A” grade plywood makes the most sense, as it will provide the best possible finish. Any “A” graded plywood should be smooth, free of knots and defects, visually appealing, color matched with no heartwood-sapwood color changes, and few (if any) mineral or vine marks. Putting a finish on “A” graded plywood requires a little extra care during the final sanding preparations, as compared to softwood plywood, because the face veneers on these plywood products are incredibly thin, often a little as 1/30th of an inch, and aggressive sanding can actually result in sanding through the face veneer, ruining your project.

Once sanded, the finish on the face side of a drawer (say in a kitchen cabinet) takes the same finish as the rest of the cabinet’s face frame. The inside plywood of a drawer requires only a couple of coats of a sealing finish (polyurethane, shellac or varnish) to protect the wood. However, finishing a drawer in a dresser requires a little more effort, particularly with the preparation of the surface. If the surface isn’t very smooth, sand with 220 or finer grit sandpaper. Even with this fine sandpaper, there may be wood fibers, which can catch on the clothing stored in the drawer. Several thin coats of polyurethane varnish, sanding between coats, will seal and protect the wood while also protecting clothing from snagging on the wood itself.

Workshop projects

While fine furniture and cabinetry is made almost exclusively from hardwood plywood, sometimes referred to as “cabinet grade plywood,” you probably won’t want to spend that much money on projects made for use in your workshop or storage room. In those cases, it is common to use softwood plywood or OSB (Oriented Strand Board); either of which would lower the price considerably, while still providing good structural strength.

These plywood materials are not normally stained and varnished, like hardwood plywood used in furniture projects is. Rather, they are painted, oiled or left plain, depending on the preference of the woodworker. Painting and oiling both provide some protection from water and chemical spills, which the wood won’t have, if it is left unfinished.

Before applying the finish

As with any other wood project, before applying any finish to projects made out of plywood, the wood itself should be properly prepared. This means filling any voids, cracks, splits or knotholes in the wood, as well as sanding the entire project smooth.

Many woodworkers have become so addicted to power sanders that they only sand the parts of a project which the power sander can reach. This is a sure sign of the difference between fine woodworking and ordinary woodworking. Anyone who wants the best possible finish on their project knows that they have to start out with sanding it well.

What you do first in the finishing process will depend a lot on the type of finish you are going to apply to the project. Generally speaking, the first thing you want to do is to sand the entire project with coarse and medium sandpaper, smoothing it out. Pay particular attention to edges and any place where a knothole has been repaired. At the same time, you’ll want to take care, as it is extremely easy to sand through the face veneer, especially on hardwood plywood.

Please note that construction grade softwood plywood may have an uneven surface when you buy it. This comes from the wood grain soaking up moisture out of the air. The open and closed grains will absorb water at a different rate, affecting how they swell. So you may need to level the surface with your sanding.

On projects which are to be stained, this is one possible time to apply that stain. If you are going to be puttying gaps, nail holes and knotholes, you’ll probably want to stain the wood before applying the putty. That way, you can putty the project to match the stained wood. But if you are not going to be trying to match them, or you are going to use a “stainable” wood putty, you’ll want to apply the putty, before staining.

If the project has any gaps, cracks in the wood or knotholes, you will want to fill them with wood putty. Use a quality wood putty, preferably a pre-tinted one that matches your intended finish. All wood putty will shrink somewhat, so you’re best off overfilling slightly, then sanding it flush with the surrounding wood. For larger gaps or knotholes, multiple applications of putty may be necessary. Wood putty can also be used to fill any end grain that is visible along the edges of the plywood.

With the project filled and stained (if necessary), it’s time to finish sanding the project, using 150 to 180 grit sandpaper. The point here is to provide yourself with a surface that is as smooth as you want the finished project to be.

Finishing

Plywood projects generally fall into two basic categories: fine furniture and workspace furniture. You will most likely want to finish these two types of projects differently, as befits the different quality plywood that you are using and the purpose of the finished project.

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Finished table, Tim Zim

Many plywood projects are built for use in workspaces, such as a workshop, laundry room, home office or crafts room. You may choose to build workbenches, worktables, shelves or desks for use in these areas. In any of these cases, the purpose of the project is to provide a flat work space or storage space.

While most workbenches don’t require the same level of care and finishing attention that fine furniture does, they do need some form of protection from changes in humidity, while remaining relatively easy to clean from glue, drips, spills, etc. At the same time, it’s nice if they can remain looking good. Finishes that produce a hard/slick surface such as lacquer, polyurethane and shellac make it more difficult to hold things in place without slipping, so they are inappropriate. Rather, a finish that penetrates and protects the wood, without providing a shiny surface, is the preferred choice.

Paint

The most common finish to use on plywood projects built for work spaces is paint. Paint is both easy to apply and inexpensive, especially since most of us have some leftover paint sitting around somewhere that we can use, making it virtually free. Properly applied, paint will seal the plywood well, making it more or less impervious to chemical spills.

When painting plywood, always use a primer/sealer before applying the paint. Plywood is inherently dry and will soak up a lot of paint on the first coat, something like double the amount normally used in a single coat. In contrast, primer/sealer dries quickly, before soaking in as much as the paint does; so it seals the wood, making it possible for additional coats of paint to stay on the surface, where they can protect it.

The big problem with trying to seal plywood products is sealing the edges. Wood always absorbs more moisture through the end grain, than it does through the surface. With plywood, that means all of the edges, as the veneers are laid at 90 degrees to each other.

Typically, woodworkers will try to seal the edges of their plywood projects with the same finish that they are using for the rest of the project. That’s fine, as far as it goes; but you’ll never get it fully sealed. The finish will continue soaking into the end grain, sealing parts of it, but probably never managing to seal it all the way. This isn’t an issue with furniture, where the end grain isn’t left exposed, but it can be for utility projects, such as workbenches, where the edge of the plywood is often left exposed.

One solution to this problem, if you are going to paint the plywood, is to apply painter’s caulking to the edges of the plywood, pressing it into the grain and smoothing it out with your fingertips. Then, when you apply the paint, it will be on top of the caulking, not directly on the wood.

Oil

Oiling a plywood project is a great way to provide moisture and stain resistance, for a minimal amount of work. Since oil and water don’t mix, when you apply oil to wood projects, the oil fills the pores in the wood and prevents water from soaking in. It also tends to lift the grain, making it stand out more and providing an attractive finish.

An oil finish soaks down into the wood’s grain very well, mostly because the molecules are very small. This helps to bring out the natural grain in the wood, something you may want to do with finished pieces, not just something that’s going to be in your workshop. Fine oiled furniture can be beautiful, with a richer appearing grain pattern than varnished wood.

Tung oil or boiled linseed oil both work well to penetrate the wood, with boiled linseed oil being the more economical choice. Both are plant-based oils, although there are important differences between the two. Tung oil will actually produce a harder finish, which will cure faster and is more water-resistant than linseed oil is. It is also a clear finish, whereas linseed oil carries a slightly yellowish tint. If, for some reason, you end up using both, perhaps because you just have a little bit of each sitting around the shop, be sure to allow the original oil to dry thoroughly, before applying the other.

In recent times, mineral oil has also been used as a finish for plywood. Thinner than either Tung oil or boiled linseed oil, mineral oil will soak into the wood better. It is a good finish for cutting boards, as it is not plant based; so it will not interact with the food and become rancid. Due to its lighter viscosity, some care must be used in applying it, especially in the cloth used, which could “pill,” leaving tiny pieces stuck in the wood. Those tiny pieces of cloth can be extremely hard to remove.

To apply an oil finish to wood, flood the surface of the wood with the oil, allowing it to sit on the surface, soaking in, for about ten minutes. Then wipe off the excess, rubbing it in, in the process. For an even richer finish, a second application can be made. Before making this second application, sand the now oily wood with fine wet-or-dry sandpaper, creating dust. Leave this dust in place, when flooding the surface with oil. That way, the oil and sawdust will mix, helping to fill the pores in the wood. This is especially useful for open-grained woods like oak and mahogany.

Danish oil

Danish oil is actually a mix of oil and varnish together. By adding oil to the varnish, the oil brings out the grain better than varnish alone will. The varnish adds in protection from chemicals, which wax alone can’t provide. These products are available both in clear and colored versions, allowing for modifications of the wood’s color as it is being finished.

Since this is predominantly an oil product, it is best applied liberally to the wood, flooding the wood, as with other oils. It should be given 15 minutes to soak into the wood, and then a second coating applied, right over the first. Once that has dried, wipe down the surface. Additional coats can be applied to provide for a more luxurious finish, but they should be limited to one per day, allowing the workpiece to dry fully between applications.

Wax

Wax is an old finish for wood. Rarely used alone today, it is usually applied over other finishes, especially varnish. However, it can be applied over any type of finish. In this case, the wax is mostly used to provide a high luster to fine furniture. Some waxes are tinted, allowing them to act as a scratch filler or to alter the color of the underlying piece of furniture.

When applying any wax finish, timing is important. If the wax is applied, then wiped down to remove excess immediately, you will get a dull sheen as it dries. The longer you allow it to dry, the higher the shine will be, when you wipe it off. For the best finish, allow the wax to dry fully, then wipe the piece down with a soft cloth.

A variety of things can be used as an applicator for wax products, but a Scotchbrite pad works very well. These hold the wax well, allowing you to work it into cracks, corners and the wood’s grain.

Polyurethane

Polyurethane varnish is probably one of the most common finishes applied to projects made out of plywood, especially furniture projects made out of hardwood plywood. Polyurethane is easy to work with and gives an excellent finish, even with minimal skill.

When using polyurethane, it is a good idea to apply a coat of sanding sealer, before applying the varnish itself. Sanding sealer acts as a clear primer, sealing the wood grain and preparing the surface for the varnish. After applying the sanding sealer, sand lightly with a fine sandpaper, about 220 grit, to clean off any “fuzz” sticking up from the sealer. Wipe off the dust before applying any coats of varnish.

For the best possible finish, apply several coats of varnish, at least three or four, lightly sanding with very fine (320 grit) sandpaper between coats. This sanding removes the bumps caused by any dust particles and gives the finish something for the new varnish to “bite” onto, ensuring good adhesion.

Once you’ve applied as many coats of polyurethane as you intend to, final polishing of the workpiece can be done with Scotchbrite finish pads. There are fine and ultrafine versions of these, which look just like Scotchbrite scrubbing pads, but are softer, with thinner strands and no added abrasives. These can either be used by hand or with a random orbital sander. The fine pad will give you an eggshell finish and the ultrafine one will provide a semi-gloss finish.

If you want an even higher luster finish than that, I would recommend waxing over your varnish. While high gloss varnishes exist, it is difficult to get a perfect finish in a home workshop, because of dust getting into the polyurethane before it dries. Unless you have a paint booth to work with, you’ll probably always find some specs of dust in the finish, perhaps not seeing them visibly, but feeling them when you rub your hand over the project’s surface. Wax will provide that high gloss shine, without giving the opportunity for more dust to settle on the project’s surface.

Combination finish

A good compromise finish for protection, penetration and beauty is a 1:1:1 mix of a good oil:varnish:thinner. This is something like making your own Danish Oil. Varnish or polyurethane both protect the wood well. Adding a thinner to this mix allows the oil/varnish mixture to penetrate deeper into the wood. To further protect your bench and give it a little extra luster, add beeswax or carnauba wax to the mixture and then buff it out once the mixture dries. Don’t add too much wax, otherwise the bench will become too slick; though adding wax makes cleaning up spills (such as glue) much easier.

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