How to finish wood: sanding grits, oil vs varnish vs wax and how to apply them

TUREFF• •0

The finish is the part people touch and see, and it is also what protects the wood from moisture, stains and wear. A good finish starts with preparation. This guide covers sanding, the main types of finish and how to apply them without streaks, dust or drips.

Sanding: work up through the grits

Sandpaper is graded by grit number; lower numbers are coarser.

Coarse (around 80): removing marks, saw lines and levelling. Only use it if you need to.
Medium (around 120): the usual starting point on planed wood.
Fine (around 180): smoothing ready for most finishes.
Very fine (around 220 and above): a final pass for a fine finish, and lightly between coats.

Rules that save time:

Do not skip more than one grade at a time; each grit's job is to remove the scratches from the last one.
Sand with the grain when sanding by hand. Cross-grain scratches show up badly once finish goes on.
Wipe a little white spirit or water over the surface to reveal scratches and glue spots before finishing (water will raise the grain; sand lightly again once dry).
Remove all dust with a vacuum and a lint-free cloth before the first coat.

Wear a dust mask while sanding, especially with hardwoods and MDF.

The main types of finish

Oil (such as linseed, tung or blended "Danish"-type oils)

Soaks into the wood and brings out the grain with a natural, low-sheen look.
Easy to apply and easy to repair: just add more oil.
Offers less protection against water and scratches than varnish.
Good for: furniture you want to look natural, turned items, and (with a food-safe product) cutting boards.

Varnish (including polyurethane, water-based and oil-based)

Forms a hard film on the surface.
Much better protection against water, heat marks and wear.
Harder to repair invisibly; a damaged area often needs sanding and recoating.
Good for: tabletops, floors, doors and anything that gets heavy use.

Wax

Gives a soft sheen and a pleasant feel.
Offers little protection on its own and needs renewing.
Often used over oil or as a final buff on a cured finish.

Shellac is another traditional option; it dries quickly and is often used as a sealer, but it is easily marked by water and alcohol.

How to apply oil

Apply a generous coat with a cloth or brush.
Leave it to soak in for the time the tin suggests.
Wipe off all the excess. Oil left on the surface turns sticky.
Repeat for several thin coats, letting each dry fully.

Fire warning: rags soaked in drying oils such as linseed can heat up as the oil cures and catch fire by themselves. Lay used rags flat outdoors until fully dry, or put them in water in a sealed metal container. Never leave them balled up in a bin.

How to apply varnish

Work in a dust-free, well-ventilated room.
Stir, do not shake (shaking makes bubbles).
Apply thin coats with a good brush or foam pad, in the direction of the grain.
Let it dry, then sand lightly with very fine paper to remove dust nibs before the next coat.
Two or three thin coats beat one thick one.

Drying times

Drying times vary widely with the product, temperature and humidity. As a general rule:

Touch dry comes first, sometimes within an hour or two for water-based products.
Recoat time is longer, often several hours or overnight for oil-based finishes.
Full cure, when the finish reaches full hardness, can take days or weeks.

Always follow the label, and go easy on a new piece until the finish has cured.

Common problems

Blotchy stain on pine: use a pre-stain conditioner or a gel stain.
Sticky oil: too much left on; wipe with more oil, then wipe dry.
Dust in varnish: sand smooth when dry and recoat.

What finish do you use, and what results have you had? Share your tips in the comments below.