What you need to know about wood finishes

From hand-cut dovetails to screw-together joinery, amateur woodworkers and DIY enthusiasts use a wide range of skills. But there's a common line every project must cross before it's done: the finish line. Finishes are necessary to protect and beautify most woodworking projects, yet the choices can be bewildering and application methods differ from one product to the next. Getting the finish right is no chance affair.

I've met many a hobbyist who had built a stunning project but was terrified to embark on the finishing journey, and sometimes with good reason. It takes just one failed attempt to make the “unfinished” look like a tempting option, but a little knowledge goes a long way.

Although the many names of finishing products can be confusing, most belong to one of four groups: straight oils, oil/varnish blends, varnish and water-based finishes. Each has its own characteristics and application methods. Most finishes fall into one of these groups, with the exception of shellac and lacquer. Look for the characteristics of each group. As for application methods, all of these products can be applied with ordinary hand-finishing methods. For the perfectionists among you, see “Finishing the Finish", page 3. With a little extra time and elbow grease, you can make the difference between a good finish and an outstanding finish.

Straight oils

The straight oils category includes finishes such as pure tung oil, raw and boiled linseed oil, mineral oil and walnut oil. These are penetrating finishes: they don't build a film on the wood. Although oil finishes don't provide any real scratch or scuff resistance, the lack of film can be an advantage. On a cutting board, for example, a film finish would crack and peel. Small bits of finish would mix with your food, raising questions of food safety.

Choose a straight oil primarily for heavy-use items that are subject to cutting and pounding. This includes cutting boards, butcher's blocks, knife blocks and workbenches. These kinds of items can be recoated with more finish as needed. My workbench is finished with many coats of tung oil, and I add another coat each year when I close my shop down over Christmas.

Straight oils are also suited for decorative tems that won't see any abuse at all, such as sculpture or turnings. Oil finishes leave the wood looking natural, with a matte finish–wonderful on “art” pieces. But they aren't protective enough for most furniture, and can really collect dirt and grime on regularly handled items.

For food contact safety on projects such as cutting boards, stick to tung, walnut or mineral oil. Walnut and mineral oil will remain greasy to the touch because they don't fully cure, but they are food-safe. Tung oil isn't pure unless the can says “pure” or “100%.” It's also food-safe; each coat needs several days to dry. Don't forget about nut allergies with respect to walnut and tung oil; tung oil comes from a Chinese nut. Where nut allergies are an issue, mineral oil is the safest choice for food contact. You can pick up a bottle of light mineral oil from your local drugstore.

Raw and boiled linseed oil are also frequently used for decorative items, or sometimes as a first coat underneath other finishes, such as oil-based polyurethane. Raw linseed oil isn't often used since it can take a full week to dry. But boiled linseed oil has added metallic driers that speed up the drying time.

Applying oil finishes is easy: simply swab on a liberal amount with a rag in any direction you want. Circles and figure eights are fine. Use a foam roller to cover large areas. Let the oil soak in for about half an hour (they all dry slowly) and then wipe off the excess with a cloth, rubbing in the direction of the wood grain.

 

 

In the case of curing oils, such as linseed and tung oil, wait until the finish dries fully, then add the next coat. It will take at least five coats to see any sort of a sheen, but you can apply as many coats as you want. For non-curing oils, such as walnut and mineral oil, you can apply a second or third coat if you want the wood to soak up more; otherwise, just one or two coats will do.

When you apply oil to cutting boards and salad bowls, the oil will disappear after several washings with soap and water, so keep a bottle of the oil under the kitchen sink and add another coat whenever the wood looks dry.

Oil/varnish blends

Danish oil, also called “oil/varnish blend,” is made by mixing a small quantity of varnish with a larger quantity of a curing straight oil, such as tung or linseed oil. It's thinned with mineral spirits (i.e., paint thinner) and metallic driers speed up drying time.

By mixing an oil finish, which is easy to apply, with varnish, which is harder to apply but more protective, you get the best of both worlds. There's more protection than a straight oil finish because each coat builds a thin film. You can add to the protection by buying an off-the-shelf Danish oil and adding varnish, such as oil-based polyurethane. I suggest a mix of 25 per cent poly to 75 per cent Danish oil. Don't add more than 25 per cent poly: the mix will be harder to apply.

An oil/varnish blend is great for everything except high-wear items. It isn't suitable for a kitchen table, for example, which will see lots of spills and abuse. But it's a beautiful finish for projects such as gently used coffee or end tables, beds, night tables or mirrors.

Applying these blends is as simple as applying straight oils: you just wipe on, then wipe off. The difference is that the finish should be left for just 10 to 15 minutes before wiping off the excess. The varnish component becomes gummy if it's left thick for too long. Allow at least 12 hours of drying time between coats; 24 hours is better. Keep in mind that colder temperatures and higher humidity increase drying time for all finishes.

Varnish

Varnish is made by heating a straight oil with resin. Natural resins exist, but most varnish is made with synthetic resins, such as polyurethane or alkyd. Even products sold as “polyurethane” are generally alkyd-modified polyurethane, which means it's alkyd varnish with poly added for extra toughness.

Spar or marine varnish is like regular varnish except the oil component is higher, making it more flexible. This is important outdoors, where humidity fluctuations make wood move. Use spar varnish where you want ultimate protection from water. UV inhibitors are included too, which prolongs the life of your outdoor furniture.

Wiping varnish is nothing more than ordinary varnish thinned with mineral spirits. When thinned, you can apply it by rag instead of by brush, eliminating brush marks. Each coat will be much thinner, so you'll need more coats. You can make your own wiping varnish by buying oil-based polyurethane and adding one or two parts of paint thinner. Make it thin enough to wipe on easily, although if it's too thin you'll have to apply more coats.

To apply wiping varnish, rub a rag with the grain direction. Work in a long stroke from one end to the other, then start the next stroke. Apply as evenly as you can and then leave it alone. Unlike oil/varnish blends or straight oils, you have to be careful about the direction in which you apply varnish because you don't wipe off the excess. Varnish dries hard in a thicker film. You can't wipe it later because the varnish sets up quickly and becomes gummy.

For ordinary brush-on varnish, including spar, thinning the product is the key to good results without defects, such as brush marks and air bubbles–these surface marks have time to level out. In my experience, the thinner you go, the more coats you'll need, but the better the results. I thin the first coat of varnish–the sealer coat that soaks into the wood and fills the pores–25 to 50 per cent. For the next two or three coats, thin the product just 15 to 25 per cent to get a quicker build-up.

Varnish should be brushed on with long strokes, moving from the middle of the board to the ends. Don't brush into the ends or you'll get a run over the edge. Once you've covered an area, don't go back unless it's just a minute later (e.g., to remove a hair) or you'll make it worse. Deal with defects later.

 

 

After each coat dries fully, sand lightly with 320-grit paper or higher (400 or 600) to remove imperfections such as dust nibs. The light scratches also give the next coat a mechanical bond. If you stained the wood prior to finishing, don't sand until after you've applied two coats of varnish or you'll sand through and remove the stain.

While three coats of varnish are usually enough, high-wear areas such as table tops call for four or five coats, especially if you're thinning the varnish a lot. I apply a minimum of three coats by brush, then I remove defects with sandpaper, then I add one final coat of wiping varnish, which goes on very smoothly.

You can use varnish on just about anything except items for cutting or pounding–the finish film will crack and flake off. Varnish excels on high-wear items. Look at the “Finish Options” table for details.

Water-based finishes

There's only one true water-based finish. If the can says, “Wash your brushes with soap and water,” then the finish is water base. These finishes are always milky white in colour and turn clear as they dry. Water-based products are marketed under different names. Sometimes they are called “water-based varnish” or “water-based polyurethane.”

As shown in the table, water-based products have excellent wear and scuff resistance, which is why they are used on wood flooring. But water-based finishes don't have the other highly resistant properties of traditional varnish. Still, use a water base for a protective finish where high-performance water and heat resistance don't matter. These finishes are also healthier to use, since they're primarily thinned with water, not a strong solvent.

Water-based finishes are unique in that they're absolutely clear. Other finishes create an amber tone. If you want a clear finish, water base is your only option. You'll also use a water base over light stain or paint, such as white, cream, peach or even pickled wood. Oil-based products add too much colour, completely changing the tone of light stains.

To apply water base, brush on the finish in long strokes using a synthetic (poly or nylon) brush. Unlike varnish, you have no time to waste. The product dries quickly, and if you over-brush you'll drag up semi-dried finish. Cover well in one stroke and move on; sand out problem areas later.

Because water-based finishes dry fast, dust nibs aren't as much of a problem. But the cured film will still benefit from a rubbing schedule after you're done (see right, “Finishing the Finish”). Quick drying time also makes brushing the finish on more difficult, so you'll really have to move fast. Mastering oil-based varnish first will get you ready to work with water-based finishes.

Handling water-based finishes is key. They are very temperature-sensitive, so don't use them under 23ºC. The air has to be that temperature, as does your project and the can of finish. If the can has been stored in a cool area, lower the entire can about 3/4" into a pail of warm water for about half an hour before you start. Never try to heat a finish on a stove or open flame.

Water-based finishes shouldn't be thinned like oil-based varnishes. They have a complex formula that allows a solvent-based finish to be dispersed in water to increase safety to the user. But it's a fragile formula that can be disturbed by excess thinning. As a general rule, use distilled water and don't thin by more than 10 per cent.

Test and have fun

As daunting as fine finishing can be, experience is the best way to learn. There are a variety of finishes, so experiment with them all. The best advice is to test your chosen finish on scrap wood first. Use the same species as your project, sanded to the same grit level. Then you'll know exactly what to expect. Happy finishing!

 

 

 

 

 

 

 

FINISH
OPTIONS
STRAIGHT
OILS

 

 

 

(includes pure tung oil, raw linseed oil,
boiled linsee oil, mineral oil and walnut oil)

OIL/VARNISH
BLENDS

 

 

 

(sometimes known as Danish oil or Scandinavian
oil)

VARNISH

 

 

 

(always oil-based, such as alkyd varnish,
spar or marine varnish, polyurethane or wiping varnish)

WATER
BASE

 

 

 

(includes latex urethane acrylic finish-sometimes
called water-based poly or water-based varnish)

APPLICATION
DIFFICULTY LEVEL
LOW LOW MODERATE HIGH
WEAR
AND SCUFF RESISTANCE
LOW
DUE TO THIN FILM
MODERATE
IF MANY COATS USED
HIGH HIGH
HEAT
RESISTANCE
MODERATE MODERATE HIGH LOW
WATER
RESISTANCE
LOW LOW
TO MODERATE
HIGH MODERATE
DUST
PROBLEMS IN FINISH COAT
LOW LOW HIGH MODERATE
HEALTH
SAFETY IN USE
MODERATE
TO HIGH
MODERATE MODERATE HIGH
AMBERING
COLOUR
MODERATE MODERATE HIGH
ESP. OVER TIME
NONE
DRY
TIME BEFORE NEXT COAT
LONG MODERATE SHORT
TO MODERATE
VERY
SHORT

 

 

Required tools

1. Tack rags: a commercially prepared rag made of cheesecloth with a varnish-like substance to collect dust. Don't use with water-based products: the oily residue will interfere with adhesion.

2. 320-, 400- and 600-grit wet/dry sandpaper: use to level a finish and remove dust nibs, either between coats or after the final coat.

3. & 4. Pumice and rottenstone: crushed lava rock (pumice), used with a lubricant, will lower the sheen of a finish. Crushed limestone (rottenstone) will increase the sheen.

5. Rags: any absorbent material will do; old T-shirts work well.

6. & 7. Paste finishing wax: apply over any finish to increase lustre and smoothness. Apply sparingly through a cotton rag and buff off excess.

8. Stir sticks: use the paint-store variety or make your own from lumber scraps. Always stir up the solids at the bottom of the can, especially with varnish.

9. Foam roller: distributes wipe-on/wipe-off finishes quickly, such as Danish oil or straight oils. Roll the finish on, but wipe off excess with a rag. Doesn't work well with varnish or water-based products (it leaves bubbles in the finish).

10. Synthetic-bristle brush: the only recommended brush for water-based finishes. Buy a good-quality one with longer bristles in the centre (chisel tip) and “split” ends

11. Natural-bristle brush: a must-have for oil-based varnish, such as polyurethane or spar varnish. Usually made from Chinese hog hair.

12. Foam brush: some prefer this to a traditional brush. Can be used with water- or oil-based products, but not with lacquer or shellac (it will dissolve).

13. Finishing pad: make your own with T-shirt cotton. Has no seams or corners, leaving an exceptionally smooth surface. Great for wiping varnish.

14. Brush/roller spinner: spins thinner out of brushes and paint rollers just prior to use or in cleaning. Spin in an empty paint can.

15. & 16. #0000 steel wool and synthetic steel wool: use to smooth out imperfections or lower the sheen after the final coat. Use super-fine synthetic pads for water-based products and regular steel wool on everything else.


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