How to Finish Wood – Varnish Or Seal?

Being involved in DIY workshops I am often asked about the difference between whether to varnish or seal a finished project. Both are excellent finishing techniques – when applied in the right circumstances.

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

Even after a synthetic finish has been applied to the surface of wood, the wood continues to expand and contract depending upon the external ambient 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 wet weather you may find that a exterior door, garage door, or even a timber gate, will swell and stick, or you may struggle to open a timber window. This is due to the fact that the wood has swollen (expanded) as a result of excess moisture being absorbed into the wood. The swollen wood will eventually dry out and revert back to its original shape, although sometimes a bit of sanding down may be required.

Taking the above into consideration, when deciding what finish to apply to your wood projects it helps to know a few facts beforehand.

What is varnish?

Varnish is a blend of resins and oils that are applied to the surface of wood to provide a protective layer against damage. Varnish is available in various formulations from gloss and semi-gloss to a matt finish. The most common varnish used today is polyurethane varnish.

Varnish does not penetrate into the wood, rather the first coat of varnish blocks the surface of wood to offer protection from ambient moisture. However, due to the constant expansion and contraction of timber, over time the varnish layer may crack and perish allowing moisture to gain access to the wood and further exacerbating the problem.

What is sealer?

Sealer is combination of resins and oils, but with a lower viscosity than varnish, which means that sealer actually penetrates into the wood itself to nourish and protect from within – rather than on top. It is essential to apply three coats of sealer to wood surfaces, as the first coat penetrates into the wood, the second coat bonds with the first layer to form a seal, and the third and final coat provides a surface that is heat, scratch and water resistant.

Sealer can be applied to almost any timber surface – interior or exterior – and there are various types of varnish specifically designed for interior or exterior conditions. UV stabilisers are added to exterior sealers to protect against sunlight and some even contain mould and fungus inhibitors. Before applying any sealer to your exterior timber be sure to check the label that the sealer meets with all your local climate conditions.

Now we know the difference between varnish and sealer it makes a little bit more sense as to the finish we should apply to a particular project. Sealers are perfect for any surface whilst varnish is better for indoor surfaces where there is little chance of high humidity or moisture. For example in a bathroom, laundry or kitchen.

Here are some tips for when using varnish or sealer:

  1. Do not wipe the paintbrush on the inside of the tin as this adds air bubbles and can spoil the finish of your project
  2. Apply sealer or varnish with the grain of the wood and work away from an edge for a much smoother finish
  3. Never apply too much or too little. If you have never done this before practise on a scrap piece of wood to get the hang of it before you start on your project
  4. Should dirt (or bugs) spoil the surface whilst it is 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 away the blemish
  5. Always wait for the first coat to dry completely before applying the second coat. A tacky first coat will spoil and you will have to sand everything down and start again
  6. For a high gloss finish lightly sand between coats, excluding the final coat

And there you have it. If you follow the guidelines above you will have a perfectly finished project that can take pride of place in your home.