When participating in DIY workshops, I am often asked about the difference between varnishing or sealing a finished project. Both are excellent finishing techniques, when applied under the right circumstances.
Did you know that the internal structure of wood contains thousands of microscopic cells or tubes that go up and down the grain of the wood? These tubes carry nutrients that are essential for tree growth and, once cut and dried, continue to expand and contract under conditions of high humidity and humidity.
Even after applying a synthetic finish to the wood surface, the wood continues to expand and contract depending on the external environmental conditions in which it is placed. It is not a process that you are going to notice, but there are some obvious indicators that this is happening. In very humid climates, an exterior door, garage door, or even a wooden door may swell and stick, or you may have difficulty opening a wooden window. This is due to the fact that the wood has swollen (expanded) as a result of excess moisture absorbed by the wood. Swollen wood will eventually dry out and return to its original shape, although some sanding may be necessary at times.
With the above in mind, when deciding what finish to apply to your wood projects, it helps to know a few facts beforehand.
What is varnish?
Varnish is a mixture of resins and oils that are applied to the surface of the wood to provide a protective layer against damage. The varnish is available in various formulations, from gloss and semi-gloss to matte finish. The most widely used varnish today is polyurethane varnish.
The varnish does not penetrate the wood, but the first coat of varnish blocks the surface of the wood to offer protection against ambient humidity. However, due to the constant expansion and contraction of the wood, over time the varnish layer can crack and perish, allowing moisture to enter the wood and further exacerbating the problem.
What is sealant?
Sealer is a combination of resins and oils, but with a lower viscosity than varnish, which means that the sealer actually penetrates the wood to nourish and protect from the inside, rather than from above. It is essential to apply three coats of sealer to wood surfaces, as the first coat penetrates the wood, the second coat adheres to the first coat to form a seal, and the third and final coat provides a surface that heats up, scratch and waterproof.
The sealer can be applied to almost any wood surface, interior or exterior, and there are several types of varnishes designed specifically for interior or exterior conditions. UV stabilizers are added to exterior sealants to protect against sunlight, and some even contain mold and mildew inhibitors. Before applying any sealer to exterior wood, be sure to check the label that the sealer meets all local weather conditions.
Now that we know the difference between varnish and sealer, it makes a little more sense as to which finish we should apply to a particular project. Sealants are perfect for any surface, while varnish is best for interior surfaces where there is little chance of high humidity or humidity. For example in a bathroom, laundry or kitchen.
Here are some tips for using varnish or sealer:
- Do not clean the brush on the inside of the can, as this adds air bubbles and can spoil the finish of your project.
- Apply sealer or varnish with the wood grain and work from one edge for a much smoother finish.
- Never apply too much or too little. If you’ve never done this before, practice with a piece of wood to get used to it before starting your project.
- In case dirt (or insects) spoil the surface while it’s still wet, leave it there. You will only spoil the finish. Wait until the coat is dry and use fine steel wool or 240-grit sandpaper to lightly sand the stain.
- Always wait for the first coat to dry completely before applying the second coat. A sticky first coat will spoil and you’ll have to sand everything down and start over.
- For a high gloss finish, sand lightly between coats, excluding top coat.
And there you have it. If you follow the guidelines above, you will have a perfectly finished project that can take center stage in your home.