Why exterior varnish should not be used to protect wooden floors and interior stairs

Exterior varnishes are semi-transparent to effectively protect wood against UV rays. We have explained this at length here. The semi-transparency is due to the pigment content which colors the varnish in wood-specific shades. The pleasing appearance leads many to choose it, even if the intention is not to use it to protect the wood outside. Recently a lady asked me if she could use such a varnish, which was exactly the antique oak color she wanted, to protect her floors and stair treads. Color is not the main characteristic of a varnish, of a protective product. Even if it has the right color, it doesn't have the right qualities to protect the floors. Floor varnishes are primarily hard and abrasion resistant, whereas exterior floor varnishes are elastic, which means low hardness. Find out why outdoor varnishes need elasticity and what are the main characteristics of materials that protect floors below.

Exterior varnish is elastic, which is not suitable for parquet and flooring

Why are exterior varnishes elastic? It all stems from wood's ability to exchange moisture with the environment until a balance is established, a process that leads to swelling or shrinkage it. The humidity outside varies widely, which will cause the moisture content of the wood to vary, with the size of the wood noticeably increasing or decreasing. If the varnish had no resilience, the movement of the wood would cause it to crack soon after application. Water from rainfall would immediately enter under the film and exfoliate the varnish, thus compromising protection.

This automatically raises two questions: why, if protected with varnish, wood still exchanges moisture with the environment and why the lake cannot be elastic and hard at the same time. The answer to the first question is because we cannot totally cover all parts of the wood. Wooden objects are made of several elements held together in different ways. No matter how hard we try, we cannot cover them perfectly and completely with exterior resistant materials (varnishes, lacquers, paints). There will always be unplugged fiber ends that will exchange moisture with the environment. Wood is only completely insulated from the outside if it is "buried" in epoxy resin, eliminating any contact with the outside.

The elasticity and hardness of varnishes are inversely proportional properties. In other words, the more elastic a varnish is, the less hard it is, and therefore less resistant to mechanical shock. Lacquers described as hard and elastic at the same time are compromises made for certain situations (e.g. some lacquers used for wooden furniture in bathroom and kitchen areas, areas with higher humidity). They cannot yet be used where the main characteristic of the varnish must be either elasticity or hardness.

The external dimensional variation of the wood - important in the radial and tangential directions and negligible in the longitudinal direction - must also be taken into account when choosing constructive solution of the object from the outside. The increase in size can cause the elements to push into each other and crack the film at the joints, creating a pathway for water to enter. One solution is to use laminated wood whose movement is greatly reduced.

Floor varnish must be hard to protect the wood from scratches and mechanical shocks

The main tests for parquet, for floors in general, are mechanical shocks (falling objects, knocks, shoe heels), the weight of furniture, heavy traffic, scratches from pets. This is why the varnish that protects it must be tough, resistant to impressions, scratching and abrasion. If you apply a soft, non-hard exterior varnish, scratches and impressions will appear immediately and the appearance will quickly become unpleasant.

However, hard varnish alone is not enough to keep parquet in good condition. The wood you choose must also be hard and abrasion-resistant. No matter how hard the varnish is, if the wood cracks, it will crack immediately. There are methods for determining the hardness of a species, the Janka test, which we have discussed at length here being the best known. Because wood is not a homogenous material, the hardness of a species is determined by an average value, as is density. Analyzing the hardness of Janka we find that the wood of fag or frasin is even more suitable for parquet than oak, and spruce or teepee should be avoided. Our hardest wood is acacia.

Exterior varnish is not a solution for refinishing floors

Exterior varnish is not suitable for parquet because the elasticity required for outdoor conditions makes it soft and without mechanical resistance. Lacquers should be chosen according to their characteristics and the area for which they are designed. There is no universal varnish that can be used with very good results for any application, just as there are no miracle products that make superb films without following the technology and working instructions. The technical factor must always take precedence over aesthetics. Only once we are convinced that the material has the necessary qualities, can we look for the color and look that satisfy us.

I hope you find the above information useful. As always, additions are welcome. And if you have any questions or queries, please leave them below in the space provided. I will be sure to reply.