What you need to know about wood finishes

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.