How UV light changes timber colour
Sunlight affects wood. Move a carpet, mat or table runner and the timber underneath will be a different colour to everything around it. Timber suppliers, prefinished flooring manufacturers and furniture makers all warn that direct sunlight should be avoided where possible. In Vic and NSW the Building Authority notes in its tolerances documents that "colour variations due to natural causes such as sunlight are not defective."
Wood reacts to UV light in the presence of oxygen. This oxidation process affects different timber species to different degrees -- the severity and speed of change varies greatly between species. UV light comes in various wavelengths. A single sheet of glass blocks almost all UVB and around 25% of UVA. Double glazing, commercial glazing systems, protective tints or UV films reduce the UV reaching the floor further. But reducing the level of light exposure does not stop the oxidation process -- it slows it down and stretches out the timeframe.
All newly sanded wood will change colour from UV exposure. Experience shows the reaction is finite and usually done within a year. The vast majority of the colour change occurs in the first 3-6 months. If something is covering the timber during this period, that area cannot react and looks different -- usually lighter -- than the exposed timber around it. If the covering is moved, the wood beneath catches up in colour, but the edges of where the covering sat often remain visible even after the rest of the timber has evened out. Unless all UV fractions are prevented from reaching the wood, changes will occur.
What the colour change looks like
A simple test: take a piece of timber, sand it back to clean raw wood, wrap one end tightly in black plastic so no light reaches it, and leave it exposed to daylight for a few weeks. When you pull the plastic off, you will see the colour difference sunlight has made.
Hardwood timbers generally darken in appearance, with different species reacting more or less strongly. Softwood timbers react similarly, though some softwood species can actually become lighter. The most common way people describe the change is that the floor has darkened, gone yellow, or taken on a more honeyed appearance.
Solvent finishes make it worse -- water-based finishes do not
Historically, floor finishes in Australia were solvent-based and contained alkyds or aromatic polyurethanes. These raw materials are well known to contribute to colour change from UV exposure. The finishes already had a yellow or honeyed shade when coated onto glass. On top of that, solvent-based finishes react to UV by becoming more yellow over time, and many become more brittle. All this extra colour stacks on top of the changes the UV causes to the wood itself, because solvent-based finishes do not block UV from reaching the timber. The end result is a dark yellow plastic look.
Bona water-based primers and finishes use aliphatic components, meaning they are not affected by UV and do not change colour or undergo any physical changes. They are categorised as non-yellowing -- they add nothing to any colour change caused by light. CSIRO testing showed Bona finishes can slow the passage of UV light through to the timber and reduce the rate of colour change. But they cannot stop UV from reaching the wood underneath, so the coated timber will still react and change colour.
This applies to all Bona coatings including Bona White. With Bona White some colour change can still be seen, because the product is not hiding the wood like paint -- it is a slightly tinted clear film that lets the character of the wood show through while also letting UV light pass through it.
Bona Craft Oil 2K or DriFast Stain provide a reasonable level of protection against UV colour change, though slight shifts may still appear, particularly with pastel colours such as Frost, White, Grey, Sand and Ash.
Bleaching from direct sunlight -- a different problem
Direct exposure to strong sunlight can bleach timber, the same way curtains bleach. With blonde timbers like Oak, Ash or Blackbutt, all colour can be stripped from the boards leaving them almost white. Species like Ironbark take on a grey washed-out tone. Red species such as Jarrah often change to a consistent light brown.
Floors coloured with Bona Craft Oil 2K or DriFast Stain have some protection against bleaching with a good degree of colour fastness. But continued exposure to strong direct sunlight over many months will still result in colour change and fading.
Steps you can take to manage it
The UV colour change is a finite reaction with the bulk of it done in the first 3-6 months. For most people the concern is not that their floor changed colour slightly, but that they can see a clear contrast after moving a mat, plant pot, or low piece of furniture.
If you can go without mats or can move things around regularly during those first few months, you can manage the issue. This prevents hard lines appearing -- the kind you get when a mat goes back on the floor a week after completion and stays put for a year. Once the colour change is broadly completed, you can place furniture, rugs and mats wherever you want without restriction.
UV films, window dressings and similar treatments help control the changes and effectively alter the timetable, but it is practically impossible to stop changes from occurring entirely.
Stopping bleaching is easier because only very direct sunlight needs to be controlled. Window dressings, external shades and reflective window films can sort out most bleaching issues.
If the UV colour changes are too large to live with, sanding and resealing the floor will give a fresh surface with no differences visible -- the reaction only affects the very top layer of the wood. You can then take steps to prevent the same thing happening again. Bleaching damage is harder to predict but in most cases a normal sand and coat will restore the floor.