What you need to know about wood finishes

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!