When wood discoloration and bleaching are required

There are situations where, in order to prepare the wood prior to finishing materials additional operations are needed, besides white grinding. This is the case of wood with unsightly discolorations or stains, either resin stains, glue stains from veneering or other previous operations. There are also situations where the wood is naturally dark in color 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 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 neutralize the surface with acetic acid 5% (vinegar). Resin stains can also be removed by washing with organic solvents.

In the case of glue stains, the stains can no longer be removed because the adhesives become insoluble after curing. In addition, where there are glue stains, the absorbency of the wood decreases. As a result, the subsequent coloration of the wood will be uneven, with light staining in these areas, and the primer will have poor adhesion to the substrate.

Wood discoloration

Chemicals are used to bleach the wood:

• not to damage the wood structure
• to act as quickly as possible
• leave no insoluble substances on the surface

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

How to bleach wood that has a darker natural color

Bleaching is a very strong discoloration that produces almost white surfaces. A mixture of the same substances as for bleaching is often used, but in different percentages and with the addition of white pigmented paste.
Maple wood, birch and plop is among the essences subject to this process.
The remains of bleaching materials sometimes influence the subsequent finish by changing its color. For this reason the supplier of the finishing materials should be informed for a proper recommendation of the materials needed for the next steps.

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