A fine finish is the crowning touch that brings out the true beauty of wood. Learn the key steps in the sanding and finishing process, and discover how to insure good results. This article offers tips for a smooth, successful finish and shows how to avoid common pitfalls.
By the DIY experts of The Family Handyman Magazine:November 2003
Sand with the grain when hand sanding or using a belt sander. Scratches are hard to see when they run parallel to the grain. But even the lightest scratches across the grain are obvious, especially after staining.
Sand curved surfaces—and other areas an electric sander can't reach—by hand. Treat all areas equally, using the same progression of sandpaper grits for both hand and power sanding.
A random orbital sander leaves scratches that are practically invisible, so you can sand across joints where grain changes direction. But move slowly (about 1 inch per second) and apply light pressure. Otherwise, you'll get swirly scratches (next photo).
Random orbital sander scratches.
Sanding always progresses from coarse to finer and finer sandpaper. Whether you're sanding by hand or using a power tool, start with 80-grit to sand away blemishes, then use 120-grit and finally 180-grit. Using these exact grits isn't vital (100-150-180 works too), but it's important to progress in steps, removing deeper scratches and leaving finer scratches each time.
You can't rely on those stain samples on display in stores. Actual color varies a lot, depending on the type of wood and how you prepared it for finishing. So save scraps from your project, run them through the same sanding process and use them to test finishes. If you didn't build the item you're finishing, run tests on an inconspicuous area—the underside of a table, for example. Test stain on scraps to get the color you want. Leaving excess stain on the wood for longer or shorter periods won't affect the color much. If it's a custom color you're after, you can mix stains of the same brand.
Water-based polyurethane affects the color only slightly. Oil-based poly has an amber tone that can dramatically change the color of stained or unstained wood. The same stain was used on the samples shown.
Turn out the lights and shine light at a low angle across the wood to reveal imperfections. Flag the problem areas with masking tape and sand them out.
Some woods absorb stain unevenly, which causes dark blotches to appear. Birch, maple, pine and cherry can all play this ugly trick on you. It's hard to eliminate this effect, but you can limit it by applying a wood conditioner before staining. Conditioner also prevents wood's end grain from absorbing more stain than the face grain. It's sold wherever stains and finishes are.
These photos will show you how to choose a finish for your project.
Usually, a brush is the best tool for applying polyurethane. For water-based poly, a synthetic brush (such as nylon or polyester) is best. For oil-based poly, use a natural-bristle brush. In either case, plan to spend a few dollars extra for a good-quality brush. Quality brushes hold more finish, lay it on smoothly and are less likely to leave lost bristles in your clear coat. If you clean your brush immediately after use, it'll serve you well far into the future.
When you notice a run, missed spot or other problem in the polyurethane you applied minutes earlier, you'll be tempted to brush it out. Don't. The finish may look wet, but chances are it's already sticky, and brushing will only make a mess. There are a couple of exceptions to this rule: You can pop tiny air bubbles with a pin, and you can pluck out a hair, a lost bristle or unfortunate fly using sharp tweezers and a steady hand.
Wipe oil-based polyurethane onto hard-to-brush surfaces using a soft, lint-free cloth. Wiping leaves a thinner coat than brushing, so you'll have to apply more coats. Water-based poly becomes sticky too fast for wiping.
Apply water-based polyurethane to large surfaces fast by using a paint pad. Water-based poly dries quickly and may not allow enough “wet” time for brushing out big areas.
Clean project and the surrounding area thoroughly before beginning the finishing process. Dust settling on wet polyurethane will give your finish the look and feel of razor stubble. So clean the area you're working in and let the dust settle. Then dust the workpiece with a soft, lint-free cloth. Don’t use tack cloth—it can leave a residue that interferes with adhesion.
When sanding between coats, it's easy to rub right through the clear coat, removing the stain below. So sand super lightly after the first coat, just enough to cut down any dust whiskers on the surface. If there are bigger problems—such as runs—deal with them after the second coat when you can sand a bit harder. To repair rubbed-through spots, just apply new stain. Immediately wipe away any stain that gets on the surrounding polyurethane.
When sanding between coats, smooth curves with a steel wool substitute such as 3M's Scotchbrite pads. Steel wool leaves fibers behind, which can cause stains in the finish.
Lightly sand between coats with 400-grit wet/dry sandpaper, which won't fall apart when it gets wet. A little water provides lubrication and keeps the finish from clogging the paper. Sanding after each coat (except the last) rubs out imperfections and roughens the surface for better adhesion of the next coat. In most cases, this is a quick job, more like wiping the surface than sanding it. When the sanding is done, wipe away the residue with a damp rag.
Always sand lightly between coats of urethane or varnish to eliminate roughness and minor imperfections and to give the next coat better adhesion. Use 180 grit or finer sandpaper and sand with the grain.
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
For water-based finishes, use synthetic bristle brushes and foam pads.
For oil-based finishes, use natural bristle brushes.
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.
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