How to Sand Wood

How to Sand Wood

Sanding wood is an ancient art, and it is one that is crucial to all forms of woodworking. From finishing a board to flattening a deck (see here) to smoothing a sculpture, sanding makes your projects shine. Ancient as it is, there is a right and a wrong way of doing it. When sanding a flat surface, start with rough-grit sandpaper and continue stepping down the grit until you reach the desired smoothness. Sandpapers run from extra-coarse grits of 24-grit, 30-grit, and 36 grit (sometimes marked as P12 to P36), coarse grits of 40-grit to 50-grit (P40 to P50), medium from 60-grit to 80-grit (P60 to P80), fine from 100-grit to 120-grit (P100 to P120), and very fine from 150-grit to 180-grit and 220-grit (P150 to P220). It's rarely worth sanding with anything finer than about P180, as finishes penetrate slightly rougher woods better and the finish becomes the smooth surface of your piece anyway. Here are the Zen-like basics of sanding wood properly.

TOOLS:

• Range of sandpaper (coarse, medium, and fine)

• Staple gun

MATERIALS:

• Wood glue

• Felt or lightweight cardboard

• Scrap 2 × 4 board

1 Make a sanding block. Commercial sanding blocks are flat on the bottom with a nice grip and padding to spread the force of your downward pressure. Glue a layer of felt or thin cardboard to a small piece of scrap 2 × 4 anywhere from 3 to 4 inches. To get a better grip on the block, cut a second, smaller piece of 2 × 4 and glue it to the top for a handle.

How to Sand Wood

2 Prepare the sanding block. Cut a piece of sandpaper of the desired grit slightly larger than your block. Lay it on the flat part of the block (grit side out), then fold the ends over and staple them to the top of the block.

How to Sand Wood

3 Start sanding a test piece of wood. Sand in the direction of the wood grain, applying light, even pressure. When the sandpaper gets clogged with particles or loses its grit, staple a new sheet over the old one. As you step down the coarseness of your sandpaper in stages, wipe away any accumulated sawdust from the board. Before the final sanding, wet the wood and then let it dry to raise the edges of the grain pattern. Use a fine sandpaper and light pressure to finish sanding the piece.

How to Sand Wood

Wood Finishes

Some modern furniture is designed to be assembled, used, and discarded. That's not the case, however, for the table with the hardwood joinery that you just spent a month slaving over, or for that heirloom antique—so you need to protect it. Finish your tabletops or other special wood surfaces with a protectant to ensure that they bring as much joy to the robot overlords of 2147 as they bring to you today.

Finishes are distinguished mostly by the solvent used as the base. You apply the finish as a liquid, but then the liquid solvent goes away, leaving the color and texture of the finish behind. Finishes lose their solvents by evaporation or reaction. In the former, the water or other emulsion liquid vaporizes, leaving just the finish itself behind. The material left after evaporation will dissolve in the original solvent, meaning that if you spill water on a water-based evaporative finish, you will lose the finish. A reactive finish, such as tung oil or varnish, changes at the chemical level while it cures. Once the finish is dry, you don't have to worry about accidental water spills. Choose based on the preferred finished look, and on your willingness to work with harsher chemicals.

Wax. Some consider wax a finish, while others consider it merely a polish to be used over a true finish, like lacquer or shellac. Paste waxes like carnauba and beeswax aren't necessarily waterproof and also don't create a hard shell to resist dents and dings, but they do make a piece shine. (Antique furniture may have remnants of an original wax finish, and it can be hard to replicate the color without applying more wax.) To apply paste wax finish, wrap a piece of cotton-based cloth around your fingers and rub on the wax as you might when polishing a pair of shoes. Rub the wax into the grain, and don't be shy about applying multiple coats.

Oil. Like wax, some woodworkers choose to use an oil finish as a topcoat after first applying a more durable, protective finish, like a varnish or lacquer. Also like wax, oil finishes are best applied with a cloth wrapped around your hand. Due to the long drying time of oil finishes—especially linseed oil, which can take three days—don't apply too much at once. Work the oil into the grain, and when you see the grain refuse to accept more finish, stop and let the coat dry.

Varnish. The durable, shiny surface of a varnish is made of resin, which is combined with drying oil and a thinner or solvent. The solvent keeps the resin in liquid form, and then evaporates to leave the hard finish. A common mistake when applying varnish is brushing it on over dust or other specks, which become permanent additions to the wood finish (until they rip free, chipping the varnish). Thoroughly clean the surface before applying varnish.

Shellac. Shellac is a versatile finish with a long history. It's nontoxic and can be used to seal wood before staining, or it can itself be mixed with colors. Basically, shellac is bug poop. The female lac bug of India and Thailand secretes a resin that dries on trees and can be scraped free in flakes. The flakes are dissolved in ethyl alcohol to make a liquid. After the liquid is applied to the wood, the alcohol evaporates, leaving your project covered in a thin, protective layer of lac bug secretion. Cool, huh? Shellac can be brushed or padded onto wood. Brushing is straightforward, but consider using a disposable brush, because it can be especially difficult to clean shellac from the bristles. To pad shellac, wrap cotton muslin around an old sock and then pour shellac into the sock. Gently squeeze the sock to force shellac into the muslin and then apply it to the wood with long, smooth strokes.

Lacquer. Lacquers come in brush-on and spray-on forms, and both dry very quickly. Lacquer is very similar to shellac, only while shellac is made from bug excrement found on lac trees, lacquer comes straight from the lac tree itself. A chemical additive cements multiple coats together, even after the first coat dries. Use a natural-bristle brush and apply a quick, thin layer first and follow with additional, thicker layers. Lacquer and polyurethane (see following) are used in much the same way, but should not be used on the same piece.

Polyurethane. This forgiving finish is available in water-based or oil-based formulas. Oil-based polyurethane requires fewer coats than water-based polyurethane but takes longer to dry and can hold the patterns of brush marks. Water-based finish requires at least three (and up to five) coats, and even then remains susceptible to water marks. That said, water-based stain won't smell up the room with toxic fumes. Water-based finish also selflevels, so it tends not to hold brush marks. When mixing polyurethane finish in preparation for application, go with the reverse James Bond—stir, don't shake, which would introduce thousands of tiny bubbles. Work with the grain and gently brush away any lingering bubbles. Inevitably, you will leave a few, but don't worry—the marks will disappear in a month.

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