Create a flawless urethane finish without a sprayer
Figuring out how to finish wood reminds me of learning math. As long as you understand the basics, the process isntoo bad. But if key bits of know-how are missing, incomplete or applied at the wrong time, misery is sure to set in. Successfully applying a basic urethane finish is the woodworking equivalent of solving simple math problems, and silky-smooth results are guaranteed to happen if you do everything right.
Belt sander, half-sheet sander, quarter-sheet sander —these are the three power tools I use most often in preparing interior surfaces for fine urethane finishes. If your project involves solid-wood panels glued together on edge, for instance, then you start with a belt sander spinning an 80-or 100-grit abrasive to level all the glue joints. If youdealing with smoothly planed, unlaminated pine, then a half-sheet sander with a 120-grit abrasive is a good start. A 150-grit abrasive is often ideal for beginning on hardwood-veneered plywood.
Why not a random-orbit sander at this stage? I like these tools for sanding where rapid wood removal is more important than super-fine results, but random-orbit sanders can sometimes leave swirl marks behind, especially on softwoods finished with stain.
The little-known half-sheet sander is better for this stage. Its pad moves back and forth, along the wood grain, with little chance for cross-grain swirls. Finish up with a quarter-sheet finishing sander fitted with 180-grit paper, then again with 220-grit paper.
Regardless of the sanding steps you follow, be sure to work with light shining off the surface at a shallow angle. Itcrucial that you switch to a finer grit of sandpaper only after the one youusing has erased all marks left by the previous grit.
When Ifinished sanding, I remove dust from the surfaces with a vacuum fitted with a round dusting brush.
Iteasier to succeed with oil-based urethanes than water-based. Drying speed is the reason why. Water-based formulations dry much more quickly than oil, so brush strokes wonflow out as easily. You still can get good results with water-based, just use a foam applicator.
Regardless of how thoroughly you sand before applying urethane, your wood will be rough after the first coat dries. All types of urethane —both oil-and water-based —cause surface fibres to swell, stand upright and harden in the upright position. The surface wonlook rough, but the wood will feel like it has a bad case of five oshadow. Sand again lightly with fine sandpaper. For large, flat areas, I use 220-or 240-grit, open-coat sandpaper in a quarter-sheet finishing sander. Use this same sandpaper by hand for edges narrower than 3″. Even something as mild as a quarter-sheet sander will eat right through the first coat or two of finish if the pad pressure is concentrated on narrow surfaces.
For smoothing rounded profiles and mouldings, a medium 3M rubbing pad or #00 steel wool works well. Ibeen testing a synthetic steel wool called Siawool. It works well where regular steel wool would leave a residue behind that would rust in the presence of water-based urethane. Leave the last coat unsanded unless you want to go one step further with the buffing process described in Smooth”below.
I always apply at least three coats of urethane on cabinets and shelves. High-wear surfaces, such as tabletops, should get five. Hendrik Varju has applied 10 coats to his extra-thirsty, end-grain tabletop featured on page 32. If youdone your work properly, the finishes youcreate will always add up to something you can be proud of.
Steve Maxwell is a woodworking expert and CHW s technical editor
Grab one of the spent pieces of 220-grit sandpaper you used for sanding between coats, wrap it around a piece of foam insulation, then rub all flat surfaces parallel to the grain gently to knock off the bumps left when dust particles landed on the surface while the urethane was wet. Next, grab a random-orbit sander, peel off the abrasive disc and press the sander down onto a fine 3M rubbing pad resting on the surface. Switch on the tool and let the machine oscillate the pad on the project surface, refining the finish. The results will be perfectly smooth in a few minutes, with a lovely satin sheen. Continue with a superfine 3M pad for glossy results that look like glass.