Varnish Over Exterior Paint: A Good Idea?

If you're looking to protect painted wood outdoors, you may be wondering if you can apply varnish on top of exterior paint. The short answer is yes, you can. Varnish can be used to protect, preserve, and enhance paint. However, it's important to note that not all paints and varnishes are compatible. The type of paint you have used will determine the best type of varnish to use as a top coat.

Varnish can protect and preserve exterior paint

Varnish is a great option for protecting and preserving exterior paint. It forms a protective layer over the paint, enhancing its durability and allowing the paint colour to show through. Varnish is especially useful for painted surfaces that are exposed to the elements, such as outdoor furniture, boats, and exterior woodwork.

Benefits of Using Varnish

Varnish is more durable than paint and provides a protective barrier that can withstand abrasion, moisture, and extreme weather conditions. It also imparts a lustrous finish to painted surfaces, enhancing their appearance. Varnish is available in different gloss options, including high-gloss, medium-gloss, and low-gloss or satin finishes, allowing you to achieve the desired level of sheen.

Types of Varnish

The two main types of varnish are water-based and oil-based. Water-based varnish is generally preferred as it does not contain volatile organic compounds (VOCs), which are considered health hazards. It is also less likely to yellow over time compared to oil-based varnish. However, oil-based varnish can be a good option for certain situations, such as providing a tough finish for exterior thresholds.

Preparing the Surface

Before applying varnish, it is important to ensure that the painted surface is clean, smooth, and free of dirt, grime, or flaking paint. Lightly sanding the surface with fine-grit sandpaper can help create a rough texture, improving the adhesion of the varnish. If there is flaking paint, use a wire brush to remove loose particles. The surface should then be wiped down with a tack cloth to remove any remaining dust.

Application Process

Varnish can be applied using a paintbrush or sprayed on, depending on the size of the project. For smaller jobs, an aerosol can of water-based varnish is usually sufficient. Hold the can about 8 inches from the surface at a 30-degree angle and spray overlapping bands of varnish to achieve a light coat. Allow the varnish to dry completely before sanding the surface again lightly with fine-grit sandpaper. Repeat this process, adding up to three more coats for enhanced gloss and protection.

Water-based varnish is best for exterior paint

Water-based varnish is the best product to use over exterior paint. Varnish is more durable than paint and imparts a lustrous finish to painted surfaces. When applied over exterior paint, varnish forms a protective layer that allows the paint colour to show through. Varnish also protects the paint from moisture and duress.

Most clear-coat products yellow over time, due in part to oil-based formulas that contain volatile organic compounds (VOCs). Products that contain VOCs are considered health hazards. Water-based varnish doesn't contain VOCs and will not yellow like oil-based clear-coat products. Water-based varnish is also sometimes referred to as water-based urethane, though the products may have slightly different formulas. They both perform the same duties.

To get a smooth coat of varnish, the underlying paint surface must be clean and smooth. If the surface is grimy or dirty, wash it off with a soapy sponge and allow it to dry. Preparation should also include sanding the painted surface lightly with 100-grit sandpaper to rough it up so that the varnish will adhere better. Varnish will not adhere properly to oil-based paint without first adding a primer. If you're dealing with an older painted surface, the paint is often oil-based. To determine whether the paint is oil-based, test it by rubbing a small area with a cotton swab dipped in alcohol. If the paint rubs off, it's likely oil-based and requires a water-based primer before applying varnish.

A smooth finish is doable with a paintbrush if you're experienced, but the best finishes are typically sprayed on. For most smaller jobs, an aerosol can of water-based varnish will do the job. Hold the can about 8 inches from the surface at about a 30-degree angle. Spray overlapping bands of varnish to coat the surface lightly. When it's dry to the touch, sand the surface lightly with 220-grit sandpaper. If the surface is carved, detailed, or has tight corners you can't reach with sandpaper, use fine-grit steel wool. Be careful when sanding corners and edges; it's easy to sand through the paint in these locations. Add up to three more coats for a deeper gloss and better protection.

Varnish can add a yellow cast to exterior paint

While varnish can be used to protect, preserve, and enhance paint, it may add a yellow cast to exterior paint. This is because most clear-coat products yellow over time due to oil-based formulas containing volatile organic compounds (VOCs). Water-based varnish, on the other hand, does not contain VOCs and will not yellow like oil-based clear-coat products.

Oil-based varnishes tend to have a yellow or amber cast that can alter the paint colour. This is especially noticeable on white and light-coloured paints, which can react with water-based finishes, drawing out substances in the wood such as tannins, dyes, or unknown substances in existing finishes, causing the topcoat to yellow. This can happen immediately, years later, or not at all, and there is no reliable way to predict yellowing ahead of time.

To reduce the occurrence of yellowing, a chemical stain and tannin-blocking primer can be used. However, the safest route is to avoid using any clear coat over bright whites and light paint colours. Instead, a water-based polyurethane topcoat can be used to seal exterior paints and prolong the life of the finish. Water-based polyurethane is suitable for latex and acrylic painted finishes, while solvent-based polyurethane should be used with oil-based paint.

Additionally, exterior paints are typically formulated to be waterproof and resilient to extreme weather conditions, so sealing is often unnecessary. However, a wood sealer can be used to prolong the life of the finish, especially for painted exterior furniture or decks. Polycrylic, a clear acrylic product with polyurethane resins, can be used to protect paint from moisture and wear, but it only bonds well with flat latex paint.

Exterior paint is already resilient to extreme weather

Exterior paint is designed to protect your home from damage caused by weather conditions and natural elements. The paint is formulated with chemicals that make it resilient to extreme weather conditions and waterproof.

The sun's UV rays can cause paint to fade quickly, dulling your home's exterior. UV-resistant paint can be used to prevent this, only fading at a rate of 2% per year, compared to standard paint, which fades at about 7% per year.

If you live in an area with fluctuating temperatures, 100% acrylic paint is the best option. It is durable and has high adhesion to the surface of your home, preventing moisture from settling into the wood and causing mould when temperatures rise.

For those who live in rainy areas, mould and mildew can be a significant problem. Mildew-resistant paint is an effective solution, preventing moisture from turning into harmful spores.

Cold weather can also impact the durability of paint. Applying a high-quality primer before painting can help to prevent chipping and cracking caused by heavy snow and icy buildup. Low-temperature paints, also known as "cold-set" paints, are designed to withstand low temperatures and contain special curing agents that promote faster drying and increased durability.

Overall, exterior paint is formulated to withstand harsh weather conditions and protect your home from damage. The specific type of paint you choose will depend on the climate you live in, but with the right materials, you can maintain the longevity and aesthetics of your home's exterior.

Sealants can be used to protect exterior paint

Sealants are a great way to protect exterior paint and can be used on a variety of surfaces, including wood, concrete, and masonry. They create a protective topcoat that shields against water damage, UV rays, and general wear and tear.

When it comes to wood, sealants are essential for preserving its beauty and prolonging its lifespan. Wood is porous, so it absorbs water easily, leading to swelling, warping, peeling, and rot over time. By sealing wood with an exterior wood sealer, you can protect it from rainwater and humidity, ensuring your deck, fencing, and outdoor furniture remain in good condition. Additionally, sealants can enhance the appearance of wood, allowing you to change its colour and texture to match your desired aesthetic.

Concrete and masonry are also porous materials that benefit from sealing. Without proper protection, they can crack, chip, become discoloured, and develop an unsightly salt layer called efflorescence. Applying a concrete sealer, waterproofer, or coat of concrete paint can prevent these issues and enhance the colour and luster of the surface.

When choosing a sealant for exterior paint, it's important to consider the type of surface you're working with. For wood, you can select from stains, coloured sealants, waterproofers, and other exterior sealers. Clear stains emphasise the natural colour and texture of the wood, while coloured stains allow you to create a specific aesthetic. If you're sealing painted wood, a clear polyurethane sealer is a good option, but be sure to test it on a small area first to ensure it doesn't alter the colour of the paint.

For exterior paint on other surfaces, such as furniture or decks, polycrylic is a recommended clear acrylic product. It contains polyurethane resins that protect the paint from moisture and duress. However, polycrylic bonds best with flat latex paint, so if you've used a high-sheen paint, you may encounter adhesion problems. In that case, a clear polyurethane topcoat is a better choice, as it is available in both water-based and solvent-based formulas. Water-based polyurethane is suitable for latex and acrylic painted finishes, while solvent-based polyurethane is designed for oil-based paints.

Frequently asked questions