Surface PreparationComments (0)
This article is from Issue 44 of Woodcraft Magazine
Lay the groundwork for a flawless finish.
By Keith Cochran
Many woodworkers think that finishing begins as soon as you pick up a brush, applicator rag, or spray gun. Not so. The first steps to achieving a beautiful blemish-free finish actually take place long before any oil, lacquer, or paint is applied to your creation. It begins as soon as you start smoothing the wood surfaces and correcting imperfections. That’s because your finish can only be as good as your surface preparation.
Finish augments a well-prepared surface and accentuates flaws on a poorly prepared one, so it’s important to remove mill marks, glue spots, scratches, dings, and other defects throughout the entire building process. It’s not hard. It just takes attention, patience, and knowing the right techniques.
In this article, I’ll share with you my approach to creating a smooth, unblemished surface that welcomes a finish of any kind. I’ll show you professional tricks that I use daily for detecting defects, repairing them, smoothing a surface, and then cleaning it of any residue. Follow these steps, and you can be confident in laying a clean, smooth, groundwork for an amazing finish.
Cleaning up excess glue
Glue squeeze-out at joints is inevitable when building projects, and drips and smears elsewhere aren’t uncommon. There are two basic methods for cleaning up glue: wiping and paring. Wiping involves cleaning up the excess glue while it’s wet. On the other hand, you can wait until the glue sets up and pare it off.
I generally wipe up excess water-based white and yellow glues during assembly. I first lightly pull the parts together with as few clamps as possible and then do an initial wipe down of excess glue. I then tighten the clamps and wipe the new squeeze-out. After adding any final clamps, I tighten everything down and then wipe one final time.
When wiping, I use clean water and a cheap synthetic abrasive pad (e.g. a Scotch-Brite green kitchen pad) for the job. Initially, make sure the pad is wet, but not sopping. Rinse it in clean water continuously as you work, refreshing the water often to prevent wiping a dilute of glue onto your surface. Wring the pad out well for your final swipes.
If clamps impede thorough wiping, wait until the glue has set enough to remove them (typically after an hour or so), and then pare away most of the still-soft glue. Just remove the heavy excess at this point (Photo A) to avoid scarring the wood. Then let the glue cure overnight before scraping and/or sanding away the remainder.
Paring can be a better approach when cleaning up at offset joint intersections, such as the junction of a leg and inset rail. Clamp the assembly long enough for the glue to safely set, remove the clamps, and then use a very sharp chisel to pare away the softened bead of glue (Photo B).
To minimize excess glue removal, you can mask off joints using blue or green painter’s masking tape. (Avoid old tape, which tends to degrade.) Tape off the parts to within 1⁄16" of their joint edges prior to glue-up. Then remove the tape when paring away the glue. After joint cleanup, check for stray drips or smudges elsewhere. To help detect them, wipe the surface with water or mineral spirits to highlight glue splotches (Photo C).
Identifying and correcting imperfections
Following glue cleanup, inspect your workpiece for dents, holes, and scratches. Scrutinize a surface under strong raking light in a darkened shop, as shown in Photo D. Direct the light from various angles to expose any defects, lightly circling them with a pencil.
Many dents are easily removed by steaming them out with a household iron and a wet rag. Wet the dented area well, letting the water soak in for a few minutes. Then place a clean, wet rag over the area, applying pressure with a very hot iron (Photo E).
Repeat every 15 seconds, reducing the moisture as the dent rises. Repeat until it has leveled out or won’t rise any more.
Fill holes with a latex or solvent based wood putty or epoxy. Try to avoid spreading filler into adjacent wood pores in open-pored woods like oak and ash. Arm your putty knife with just a dab of filler, and carefully target the hole (Photo F).
Alternatively, you can mask off the area around the hole.
Remove deep scratches by scraping and/or sanding. A sharp cabinet scraper is quick and effective for this, but sandpaper alone can be used. Start with the finest grit that will cut away the scratch, and then move through successively finer grits. To avoid creating a depression, feather outward from the scratch as you work.
Smoothing the surface
After fixing defects, smooth your work. For an impeccable finish, you need to remove any milling marks, planer snipe, and machine burns. You can use scrapers and planes for some of this work, but I do it exclusively by sanding.
For efficiency, I use several sanders, each equipped with a specific grit (Photo G). For solid wood, I usually begin with 80 grit to remove machine marks. Once I have attained a consistent scratch pattern, I move on through 100 or 120 grit, finishing up with 220 grit, while working in a slow, deliberate manner. To eliminate any sander swirls, I follow up by hand-sanding with 220 grit in the direction of the grain. I back my paper with a sanding block and work in raking light, as shown in the lead photo on page 42. Finally, I ease the corners with 220 grit for comfort and aesthetics.
For plywood, I start with 120 grit to avoid abrading through the thin veneer. Even then, I don’t overdo it, stopping as soon as I’ve attained a consistent scratch pattern. I then finish off with 220 grit, again hand-sanding with the grain to eliminate any swirl scratches.
Always work with fresh sandpaper. When it no longer feels sharp to your fingers, replace it. But don’t throw away worn sanding discs immediately, as they can be useful for sanding detailed edges, curves, and corners.
By folding a disc in half, you can appropriately use its straight edge, rounded edge, or pointed corner to smooth beads or other complex areas (Photo H).
It’s important to do one final careful cleaning before finishing. I begin by removing most of the dust with a soft-bristle bench brush, and then I take the work outside and thoroughly blast it with compressed air. To prevent the nozzle from spitting oil or water, I cover it with a clean rag made from an old T-shirt (Photo I).
Also sweep and vacuum your finish area to prevent ambient dust from contaminating your finish. Before moving your work to the area, inspect it one last time under raking light. If a piece is easy to carry I’ll also take it outside and hold it at an oblique angle to the good ol’ Alabama sun as a final check in natural light.
If you follow all these preparation guidelines, you’ll be poised to produce a finish that might cause you to proclaim to your wife and buddies, “Yeah, I made that.”
About Our Author
Keith Cochran grew up building in his dad’s home workshop and went on to earn a degree in industrial design. His background in wooden boat building and furniture design and fabrication serves him well today as he builds fine custom furniture in northern Alabama. For more information about his work, visit woodstudio.com.
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