How to Apply Varnish: Preparation, Technique and Common Mistakes

01

Preparing the Surface

Varnish bonds to wood far better when the surface is clean, smooth and free of old finish residue. Preparation usually involves:

• Sanding the wood with progressively finer grit sandpaper to remove roughness, old finish and surface imperfections.
• Wiping away all dust with a dry cloth or a tack cloth, since any dust left on the surface will become trapped under the varnish.
• Repairing any cracks, holes or gouges with appropriate wood filler before sanding the repaired areas smooth.
• Allowing the wood to dry fully if it has been washed or if it was previously exposed to moisture, since trapped moisture can cause bubbling or poor adhesion.

On wood that has never been finished before, raising the grain slightly by wiping it with a damp cloth and then sanding again once dry can help produce a smoother final coat, since it deals with fibres that would otherwise swell once the varnish is applied.

02

Choosing Tools

A good quality brush, suited to the type of varnish being used, usually gives better control than a roller for furniture and detailed woodwork, while rollers or pads can be more efficient for large flat areas such as floors. Natural bristle brushes generally work well with oil based varnish, while synthetic bristle brushes are usually recommended for water based varnish, since natural bristles can absorb water and lose their shape.

03

Applying the First Coat

The first coat is often thinned slightly, following the manufacturer's instructions, so that it can soak into the wood and create a good base for the coats that follow. It should be applied in thin, even strokes, working in the direction of the wood grain. Overworking the varnish, by brushing back and forth too many times, can introduce air bubbles and uneven texture, so it is generally better to apply it smoothly and then leave it alone to level out on its own.

04

Drying and Sanding Between Coats

Each coat needs to dry fully before the next is applied, and drying times vary considerably between oil based, water based and other formulations. Once a coat is dry, lightly sanding it with fine grit sandpaper, then wiping away the dust, helps the next coat bond properly and removes any small imperfections such as dust specks or brush marks. Skipping this step is one of the most common reasons a finish ends up looking cloudy or uneven.

05

How Many Coats Are Needed

Most projects benefit from at least two to three coats of varnish, with floors and heavily used surfaces often needing more for adequate protection. Each additional coat increases durability and depth of colour, but only if each layer is allowed to dry and is lightly sanded beforehand.

06

Common Mistakes to Avoid

• Applying varnish in direct sunlight or in very hot conditions, which can cause it to dry too quickly and form bubbles or an uneven film.
• Working in a dusty area, which leads to specks trapped in the finish.
• Shaking the varnish can vigorously before use, which introduces air bubbles. Stirring gently is usually recommended instead.
• Applying coats too thickly in an attempt to save time, which often results in drips, wrinkling or a tacky surface that never fully hardens.
• Putting furniture or flooring back into full use before the varnish has properly cured, not just dried to the touch, which can leave lasting marks or dents in a still soft finish.

07

Caring for a Varnished Surface Afterwards

Once cured, a varnished surface generally needs only regular dusting and occasional cleaning with a mild, damp cloth. Harsh chemical cleaners and abrasive scrubbing pads can dull or scratch the finish over time. For outdoor wood, periodic recoating is usually necessary, since ultraviolet light and weather exposure gradually break down even the most durable exterior varnish, and catching this early with a fresh coat is far easier than stripping and refinishing a badly weathered surface.