When wood discoloration and bleaching are required

There are situations where, in order to prepare the wood before application finishing materials additional operations are required, besides white grinding. This is the case for wood with unsightly staining or stains, either with resin stains, glue stains from veneering or other previous operations. But also situations where the wood is naturally dark in colour and the technology requires a very light, almost white surface. In all these cases, the wood must be bleached and bleached.

How to remove stains on wood

In wood with a high tannin content, stains may appear on contact with metals and can be removed by washing with weak solutions of organic acids (citric, oxalic) followed by washing with warm water.

Resin or adhesive stains can be removed by brushing with hot weak alkaline solutions, then neutralising the surface with 5% acetic acid (vinegar). Resin stains can also be removed by washing with organic solvents.

In the case of glue stains cannot be removed because adhesives become insoluble after curing. In addition, in places where there are glue penetrations, the absorbency of the wood decreases. As a result, the subsequent staining of the wood will be uneven, with light-coloured stains in those areas, and the primer will have poor adhesion to the substrate.

Wood discoloration

Chemicals are used to bleach the wood:

• not destroy the structure of the wood
• act in the shortest possible time
• do not leave insoluble substances on the surface

The most commonly used is a mixture of perhydrol and an ammonia solution. Both are volatile substances that leave no residue after use. As they are also corrosive at the same time, they must be used with great care and protective materials.
Discolouration is very rarely achieved in a single step, and sometimes several consecutive wetting steps are needed to achieve the desired result. And after staining, the wood should be left for 48 hours before finishing.
Discoloration and bleaching of wood is most common in beech, cherry and oak.

How to bleach wood that has a darker natural colour

Bleaching is a very strong discolouration which results in almost white surfaces. It is common to use the same substances mixed together as for bleaching, but in different percentages and with the addition of white pigmented paste.
Maple wood, birch and plop are among the essences subject to this process.
Remnants of bleaching materials sometimes influence the subsequent finish by changing its colour. For this reason the supplier of the finishing materials should be informed for an appropriate recommendation of the materials needed for the next steps.

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