Discolouration of timber can occur with both raw and coated timbers. However, the degree of change will depend on the light intensity and the species of the timber.
Discolouration faults fall into two kinds of causes and effects, Reddening and Yellowing – Fading.
Most timber species contain naturally occurring tannins. Under certain conditions these tannins undergo a chemical change and then develop as irregular reddish or purple patches on the underlying substrate.
Reddening of the wood occurs with:
- Acid catalysed coatings
- Urea formaldehyde glues
- Some fast-curing PVA adhesives
- Other coatings with a high acid value
Not all timbers are susceptible to reddening and the fault has mainly been encountered with Tawa, Beech, blackwood, Cherry and Maple timbers. Note: Testing is recommended for other timber substrates prior to full-scale production.
Reddening may not appear until several weeks after the manufacture of the timber substrate. This reddening effect is particularly common on timber veneers with a high adhesive coating weight. Excess glue can be forced through the grain of the veneer to near the surface of the timber veneer.
Reddening associated with the use of acid-catalysed coatings is generally less intense and more uniform in distribution than adhesive induced reddening.
However, it should be mentioned that of the timbers with a natural red tone, the application of a clear coating system would enhance the timbers natural discolouration.
Yellowing – Fading
Clear coatings offer limited protection to timber with regards to yellowing and fading. This is because the coating is transparent and allows ultraviolet light to penetrate through to the timber surface discolouring the timber substrate, causing:
- Fading or bleaching in dark timbers
- Yellowing or darkening in light timbers
It is possible to put barriers in place to reflect or to absorb sunlight however these only offer temporary protection. UV light absorbers capture UV light, but gradually become less effective as the UV absorbers have been utilised.
Light induced discolouration occurs not only in timber, but also on many items including fabrics, carpets, curtains and furnishings. It is a fundamental facts that all articles subjected to light, change colour and appearance in some way as they age. Note: Surfaces closest to a light source is affected more rapidly that the area furthest away.
The degree of colour change on clear coated timber articles varies with the amount of light to which the coating and timber substrate are exposed.
So how can you minimise Reddening and Yellowing?
- Before undertaking the construction of a timber article, consider carefully that the species of timber chosen is suitable for the intended environment and end use application.
- Select only those veneer boards where the adhesive coating weight used is the minimum required to glue the veneer to the base board.
- Stain the timber substrate with MIROSTAIN 2616 before finishing. Although this modifies the natural appearance of freshly exposed timber substrates, the resulting colour is more stable.
- Use clear coatings with low value. Contact Us to discuss with Mirotone sales or Technical for more details.
- Before undertaking the construction of a timber article. Consider carefully that the species of timber is suitable for the intended environment and end use application.
- Apply a very thin wash of MIROSTAIN 2616 Wiping Stain onto the substrate, this puts a barrier of pigment (although very thin) at the interface of the timber and the subsequent coating system. This reduces the penetration of ultraviolet light.
- Use a clear coating system formulated with UV absorbers.
- Shade clear coated furniture or panelling from direct sunlight.