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Without some kind of finish to protect it from the elements, wood can become as dry and lifeless as day-old toast. Coating wood cabinetry, furniture, or trim with a clear finish, whether you stain it or not, gives it richness and depth while protecting it from knocks, scrapes, and the weather. Use this guide to learn more about choosing and using polyurethane, the toughest of the clear coatings. Then you can be sure you're using just the right finish to bring out the beauty of wood around your house.
It's a kind of super-tough varnish formulated so that its microscopic chains of resin molecules will bond tightly with one another as it dries. The result is a finish that's much more resistant to water, solvents, abrasion, and impacts than traditional varnishes.
Restrictions on VOC emissions have spurred the development of alternatives to oil-based polys. Consider the different characteristics of each type before you buy.
Choose whichever sheen you like best; there's no difference in durability. Just remember that the glossier the finish, the more it will show any underlying imperfections and any future wear and tear.
Most exterior polys can be used indoors, but interior polys should never be used outdoors; they lack the additives that protect exterior finishes from UV rays.
Every poly has its preferred applicator, typically a brush or cloth. Some polys also come in aerosol spray cans.
Each polyurethane is unique, thanks to tweaks in its chemical makeup; choose the one that best matches your particular project.
GOOD FOR: Cabinets, floors, furniture, and trim such as wainscot, where abrasion resistance and durability are important. This versatile all-around poly provides good protection on a variety of wood surfaces. Available in gloss, semi-gloss, and satin.
APPLY WITH: natural-bristle brush or a can of aerosol spray
NUMBER OF COATS: 2-3
HOURS BETWEEN COATS: 4 to 6
VOCs: 450 grams per liter
APPLY WITH: natural-bristle brush
NUMBER OF COATS: 2
HOURS BETWEEN COATS: 4 to 6
VOCs: 350 grams per liter
GOOD FOR: Doors, cabinets, furniture, and floors. Combines the durability and ambering of an oil with the fast drying time, low VOC content, and easy cleanup of a water-based product. Available in gloss, semi-gloss, and satin.
APPLY WITH: synthetic-filament brush or a can of aerosol spray
NUMBER OF COATS: 3
HOURS BETWEEN COATS: 2
VOCs: 275 grams per liter
GOOD FOR: Light-coloured woods and stains where ambering would be undesirable. Blended with acrylic resins, it goes on milky but quickly dries crystal clear. Not as durable as oil-based polys. Available in gloss, semi-gloss, and satin.
APPLY WITH: synthetic-filament brush or a can of aerosol spray
NUMBER OF COATS: 3
HOURS BETWEEN COATS: 2
VOCs: 275 grams per liter
GOOD FOR: Carved, embossed, or profiled surfaces where a brush could leave drips. Also useful for hiding superficial scratches in previously finished wood. Its thin film offers moderate protection from abrasion. Available in gloss and satin sheens, and in oil-based and water-based formulations.
APPLY WITH: lint-free cloth
NUMBER OF COATS: 3
HOURS BETWEEN COATS: 2 to 3
VOCs: 580 grams per liter for oil-based;
VOCs: 275 grams per liter for water-based oil-modified
GOOD FOR: Furniture, cabinets, trim. Stains and protects bare wood with each coat. Before applying to bare wood, use a wood conditioner to ensure even colour. Smooth between coats with 0000 steel wool. Available in satin and gloss.
APPLY WITH: natural-bristle brush
NUMBER OF COATS: 2
HOURS BETWEEN COATS: 6
VOCs: 450 grams per liter (275 in Southern California)
GOOD FOR: Exterior doors, trim, and furniture. Contains UV absorbers that protect the finish and the wood from the sun's rays. Made with a special blend of oils and resins that allows it to flex as the wood surface expands and contracts. Recoat the finish when it turns dull, typically once a year. Available in gloss, semi-gloss, and satin, and in oil-based and water-based formulas.
APPLY WITH: natural-bristle brush or a can of aerosol spray
NUMBER OF COATS: 3
HOURS BETWEEN COATS: 4
VOCs: 450 grams per liter
In addition to reading the directions on the can, keep these basics in mind.
Acetone-based nail polish remover softens lacquer. Water drops turn wax white in about 10 minutes. If either lacquer or wax are present, strip them before proceeding.
Stir the can. Do this before and during application to evenly blend the ingredients that control sheen and UV resistance.
Apply thin coats. Thick ones take longer to dry and are more likely to drip or wrinkle.
Sand between coats. Using 220-grit paper helps ensure good adhesion and smooths away imperfections.
Wipe after sanding. Dust interferes with adhesion and leaves unsightly bumps in the finish. Use a cloth dampened with mineral spirits on oil-based finishes; use a water-dampened cloth on water-based finishes. A dry microfiber cloth also works well with both finishes.
Shaking creates bubbles, which leave a rough, pitted surface. It's okay to shake wipe-on polys, however.
Thin the first coat. Thinning doesn't improve adhesion, and you'll have to apply more coats and use more solvent.
Use tack cloths. Some contain chemicals that prevent good adhesion.
Wax the finish. Wax isn't durable and will interfere with any subsequent coats.
Use natural bristle for oil-based finishes and synthetic filament (polyester, nylon, or a blend of the two) for water-based and water-based oil-modified finishes. Don't use rollers or foam brushes—they create bubbles.
Your brush will be easier to clean, and will go longer between cleanings, if its bristles are dipped first in mineral spirits, if you're using an oil-based poly, or in water, if the finish is water-based. Before using the brush, rapidly roll the handle between your hands inside a cardboard box to eliminate excess.
Working from a separate container prevents the dust and other particles picked up by the brush from contaminating the finish in the can.
After dipping about one-third of the brush into the finish, gently slap it against the inside of the container. Scraping the bristles over the edge of a container leaves the brush too dry to apply a coat of the right thickness.
It's the best way to work the finish into the wood pores and ensure an even appearance.
Wipe the wood with a damp cloth to raise the fibers; after the surface dries, knock them back with 220-grit sandpaper.
Too much brushing can leave these fast-drying polys rough and ragged.
It leaves behind tiny bits of steel that you can't see—until they rust. Use sandpaper instead.