How to Sand and Smooth – Woodworking Techniques
Sanding and Smoothing
A finish is only as good as the surface beneath it, and getting that surface just right takes a lot of time and a little bit of know-how. The kind of wood you use makes a difference, but the way you work it is just as important.
Before you begin sanding, take a look at the wood in a good light. It should be flat and the surface smooth before you can safely apply a coating. Anyruns or forced–out material should have been cleaned off with a damp cloth before they dried. If not, they can be trimmed off with a sharp chisel.
You should be aware that some manufactured wood pieces — moldings and trim especially — may be contaminated with silicone oils or waxes left there as cutter-head lubricants. These should be removed with a cloth soaked in mineral spirits, followed by an ammonia wash of one part clear ammonia to fifteen parts warm water, spread evenly over the entire surface, then wiped dry. Waxes and oils are usually ok with oil-based finishes, but can mottle water-based finishes and lacquers; that’s why they must be removed before going any further.
Here’s a quick lesson on sandpaper, or more properly, coated abrasive. It’s made of tiny pieces of mineral grit glued onto some kind of backing. The grit determines how fast a paper will sand, and how much effort will be required to do it. The backing — and theor “binder” used to hold the grit in place — establishes how the sheet will stand up to wear and solvents.
There’s not a whole lot to understanding how sandpaper works. The abrasive removes tool marks left in the surface of the wood and replaces them with grooves established by the size of the grit on the paper. Few large grooves are turned into many small grooves as a progressively finer grit size is used.
The terms “open coat” and “closed coat” are used to describe the amount of grit on the backing. An open coat has only about 50% to 70% of the surface covered with mineral. This has the effect of periodically de-clogging the paper, because the open spaces provide a place for the cuttings to lodge until they fall off. A closed coat is completely covered with sand, which offers a finer finish. In higher-grit (extra-fine) papers, the difference doesn’t amount to much because the rough work has already been done. But with the lower-numbered grits, the open-style paper makes less heat and conserves material.
Grits can be either synthetic or natural. Though there are many different kinds of sandpaper, only a few are really practical for woodworking. Among the synthetic grits, aluminum oxide is the most common, and probably the one you’ll use. It’s light brown or white in color, and is ideal for general sanding and finishing work. For finishing by itself, another synthetic, silicon carbide, which is bright black, is generally recommended.
Garnet is a reddish mineral used a lot in sandpaper, even with the popularity of synthetics. It has the unique characteristic of breaking off as it works, presenting a new sharp facet with each fracture. Garnet paper does an excellent job but wears faster than the synthetics.
The backing used with sandpapers varies according to the tool the paper is going to be used on. Sheet papers and polyester cloth backings are both used, the latter more on belt sanders. For hand-held palm sanders, a paper backing with pressure-sensitiveis common.
When sanding, always work from coarse to fine grits. You should begin with an 80 or 100 grit, and then progress to the next finer level, increasing the point range with each step. Using this rule, a grit progression of 80/120/220/400 would be typical.
Working with too heavy a grit size results in a lot of effort with no progress, because you’re simply creating grooves. On the other hand, using too light a grit doesn’t remove the deeper marks from the previous paper, so they’ll eventually show up in the finish. Somedon’t require you to sand finer than 180 grit. The harder woods, however, may need to be taken down to a 400-grit or higher level. In general, a 220-grit sanding will give the best results, especially if you plan on using a water-based finish.
Besides the motor-driven palm sander, you might use a sanding block to achieve a flat surface and get a better feel for your work. You can also fold a paper sheet several times to do detail work or stiffen the paper in your fingers. Once you reach a 150-grit or finer level, you can begin sanding in any direction — not just with the grain — because the scratches you create will probably not be visible.
The most common error when using a palm sander is scrubbing it across the wood’s surface too rapidly. You must remember that an oscillating sander is already moving at a rate of nearly 14,000 oscillations per minute, and you don’t need to speed it up. Your hand with the sander in it should move about one inch per second to avoid leaving swirl marks in the surface.