Secrets for applying a simple, slick wood finish, every time.
Written and Photos By Steve Maxwell
Creating a silky-smooth, transparent finish on interior woodwork is a lot like opening a combination lock. You’ve got to dial in all the correct numbers before succeeding. And, while it’s true there are many ways to finish wood, there’s no need to get complicated. The urethane-based approach I’ll reveal here is simple, reliable and offers outstanding results. Success is a matter of cause and effect, beginning with an initial operation that’s often misunderstood.
Key Step#1: Correct Sanding
This is about more than just making wood feel smooth. It’s also about making it look good. Only properly sanded wood fully develops a rich appearance after finishing. Inadequate sanding creates a dull, muddy look, and the key to avoiding this pitfall involves working in stages.
Even smoothly planed wood must be sanded before finishing – every square inch of it. Don’t be fooled by the fact that the surface doesn’t feel rough. Regardless of what your finger tips tell you, it’s always got at least a fine pattern of marks left behind by the thickness planer or moulding machine that did the milling. Sometimes this pattern is quite pronounced. If you simply apply a finish without sanding, this defect becomes even more evident.
For flat pieces of softwood that appear reasonably smooth, begin sanding with a hand-held belt sander spinning a 120-grit belt. If the surface has visible mill marks, or you’re working with hardwood, choose a 100-grit belt. Only sand parallel to the grain, and evenly across all areas of the wood. Shine a floodlight across the surface of the wood at a shallow angle as you work (I use a 500-watt quartz halogen lamp) to reveal any areas you’ve missed. This strategy is amazingly helpful.
While it’s true that belt sanding creates an even surface, it won’t be smooth enough on its own for an optimal finish. To make this happen you need to make use of one of two different sanders, depending on the wood you’re working with and whether or not you’ll be using dark stain.
For applications where you’ll be using urethane only (or urethane and a very light stain), a 5″ random orbit sander is perfect for the next steps. Work over the whole surface with a 120-grit disk first, repeating the process with a 180-grit disk, using light hand pressure.
If you’ll be staining your wood, especially using a dark colour, the use of a random orbit sander can cause problems. The action of this kind of sander sometimes leaves behind irregular swirls on the wood surface that show up noticeably after staining, especially on softwoods. To avoid this trouble, use a vibrating pad sander rather than a random orbit machine. Progress will be slower, but the results will be swirl-free.
If the surface you’re working is large, use a 1⁄2-sheet model to cover ground efficiently. A 1⁄4-sheet sander is ideal for smaller areas and for sanding between coats of urethane, as I’ll explain next. In either case, start with 120-grit sandpaper, and then switch to 180.
Are you dealing with curved surfaces, trim and moulding? These need sanding at least as much as flat surfaces do, though you can’t use conventional sanders because of the wood shape. To avoid the drudgery of hand sanding, and the spotty results.
Key Step#2: Product Choice and Application
Dust is your enemy when it comes to any kind of wood finishing, but especially so when creating a surface film like you’re doing here. This is why vacuuming all areas diligently after sanding is so important.
You’ll also find it easiest to get good results with a satin finish, oil-based urethane, as opposed to a gloss. The duller sheen level of satin masks surface imperfections while the gloss accentuates them. Regardless of what you choose, be sure to apply several coats of the specific brand you’ll be using, applied to test samples of sanded wood. Some urethanes dry poorly and leave behind sticky residue, causing serious problems. The testing process ensures you avoid the hassles of dealing with a troublesome product when it really matters on your final project.
Water based urethane is an option, too, but you need to understand something. Almost all brands are designed to dry very quickly – a feature that’s proclaimed loud and clear to promote sales. Trouble is, when urethane dries quickly, it doesn’t always have a chance to flow out properly before hardening, leaving a visible and ugly texture of brush marks and bubbles behind. There is a way to enjoy the low odour and easy clean-up of water based urethanes without sacrificing finish quality by following the buffing procedures you’ll find later in step #3.
Regardless of which kind of urethane you’re using, brush on one coat, completing the application in a given area by moving the brush only in the direction of the wood grain. Let the coat dry completely (give it at least 12 hours for standard oil-based urethanes and a few hours for water based), then sand the area lightly with 240-grit paper to remove the rough wood fibers raised by the urethane. Go ahead and use your 1⁄4-sheet palm sander on large, flat areas, but switch to hand sanding around corners and in tight quarters. Edges are particularly vulnerable to over-sanding and going right through the finish into bare wood. Be gentle. It only takes the slightest amount of sanding to remove raised grain, so there’s no need to go crazy. When you think you’re finished, run your hands over all surfaces. This is the best way to detect rough areas that you missed after the wood is finished.
Vacuum everything again, and repeat the coating and sanding process. Two coats work well for light-duty situations. Three coats are necessary where water might regularly come in contact with the wood, and four coats if you’ll be going on to create the ultimate wood finish using the buffing process described next.
Key Step#3: Buffing
At this stage you’ll have a very good finish, but it won’t be perfect. No matter how well you vacuumed the surface, airborne dust will have settled onto the wet urethane, leaving tiny, hardened bumps behind. Also, if you’ve used water based urethane, chances are excellent that you’ve got at least a few hardened air bubbles and some brush strokes marring the surface. Not to worry, though. Things are about to get dramatically better.
Make sure you have four coats of finish in place, then take a piece of previously used 240-grit sandpaper, wrap it around a block of Styrofoam, then gently rub wood surfaces in the same direction as the grain. This removes all dust bumps and bubbles, and it only takes a few strokes in each direction to get the job done. Use your fingers to decide when you’ve sanded enough, then grab a random orbit sander and a fine, 3M rubbing pad. You’re about to be amazed.
Specialty woodworking retailers and auto body supply outlets all carry 3M rubbing pads. They’re non-woven, synthetic and charged with varying sizes of abrasive particles. Lay a 6×6″ square of 3M pad onto a flat section of wood, put your random orbit sander on top (with no sandpaper in place), then switch on. The sander jiggles and rotates the rubbing pad, causing it to polish the wood surface, creating an even, matte sheen. If you want a gloss shine, repeat the process with a superfine 3M pad.
The sander-based buffing process works only on flat, open areas, but you can achieve the same thing on small spaces, moulding and trim using the 3M pads by hand. Depending on the state of your finish after application, you may want to begin with some light hand sanding using 240-grit paper.
Follow all the details, and successfully finished interior wood is guaranteed.