How to finish wood – varnish or seal?


Participation in construction seminars I am often asked about the difference between whether varnish or seal the finished project. Both are excellent finishing techniques – when applied in the right circumstances.

Do you know that the internal structure of wood contains thousands of microscopic cells or tubes that run up and down the grain of the wood. These tubes carry the nutrients necessary for the growth of the trees, and once cut and dried, they continue to expand and contract under conditions of high humidity.

Even after synthetic finish has been applied to the surface of the wood, the wood continues to expand and contract depending on the external environmental conditions in which it is located. This is not a process that you are going to notice, but there are some obvious indicators that this is happening. In very humid weather, you can see that the outer door, garage door, or even a timber gate, swell and stick, or you can struggle to open window timber. This is because the wood has swollen (enlarged) as a result of the excess moisture absorbed into the wood. The swollen wood dries and to return to its original form, although sometimes a little sanding down may be required.

Given the above, when deciding that you are ready to apply to the project tree helps to know in advance a few facts.

What is varnish?

Varnish is a mixture of resins and oils, are applied to the wood surface to provide a protective layer from damage. Varnish is available in different formulations, from gloss and semi-gloss matte. The most common lacquer used today is polyurethane varnish.

Varnish does not penetrate into the wood and the first coat of varnish blocks the surface of wood to protect from moisture of the environment. However, due to the constant expansion and contraction of wood, over time the varnish layer can crack and perish allowing moisture to gain access to the wood and further exacerbating the problem.

What is the sealer?

The sealant is the combination of resin and oil, but low viscosity than lacquer, which means that the sealer actually penetrates into the wood to nourish and protect from the inside out, not the top. It is necessary to apply three layers of sealant to wooden surfaces as the first coat penetrates the wood, the second coat bonds with the first layer, forming a seal, and the third and final layer provides the surface heat, scratch and water resistant.

Sealant can be applied to almost any timber surface – interior or exterior, and there are different types of varnish specifically designed for interior and exterior conditions. UV stabilizers are added external seals to protect from sunlight, and some even contain inhibitors mold and mildew. Before applying any sealant to your exterior timber be sure to check the label that the sealer meets all of your local climatic conditions.

Now we know the difference between a paint and sealant that makes a little more sense as to the finish we should apply to a specific project. Sealers are ideal for any surface whilst varnish is better for surfaces in areas of high humidity or moisture. For example in the bathroom, Laundry or kitchen.

Here are some tips when using varnish or sealer:

  1. Do not wipe the brush on the inside of the tin as this adds air bubbles and can damage the finish of your project
  2. Apply sealant or varnish from the wood and work away from the edges to a much smoother finish
  3. Never apply too much or too little. If you’ve never done this before practice on a scrap piece of wood to get the hang of it before starting on your project
  4. Should the dirt (or errors) spoil the surface while it is still wet will leave it there. You will only spoil the finish. Wait until the coat dries and use thin steel wool or 240-grit paper to lightly sand away the blemish
  5. Always wait for the first coat to dry completely before applying the second layer. Sticky first layer will spoil and you’ll have to sand everything down and start again
  6. For glossy surfaces lightly sand between coats except the last coat

And there You have it. If you follow the guidelines above, you will have turnkey project, which can take pride of place in your home.

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