What do you do so that the tannin in the wood does not change the colour of the wood or paint

With all the attention paid to staining, irregular reddish-yellow patches sometimes appear on the surface of the wood. The lighter the colour or whiter the wood, the more visible the phenomenon. If the finished object is made of oak, chestnut or mahogany, and the bath used is water-thinnable, it is most likely the tannins in the wood. They are guilty of staining the wood, changing the colour of the varnish or paint and sometimes even slowing the drying of alkyd or oil-based products and slowing the curing reaction of catalyst varnishes. I recently had a conversation with someone who had such a problem on an oak floor. As the phenomenon doesn't always occur, giving the impression that either the varnish or the craftsman is to blame, I think it's worth discussing it a bit.

What tannins are and where we find them

Tannins are found in many plants and are a protection against insects and animals that can harm them. They are not in a fixed amount in every plant and are a fairly large and not identical mixture of substances in every plant. They are considered to give acids, for example, astringent taste of wine - arrived there from the woody part of the bunch, from the oak barrel or from pieces of oak added to wine - or softens animal skinin the tanning process. In fact, this is where the name comes from, as oak bark was often used in the past for tanning leather. Tannum - oak bark in Latin - is the basis from which both the word tannin and tanning were formed.

All trees contain tannin, both in the root (the largest amount) and in the trunk, branches and leaves. The colour of the tannin ranges from reddish-yellow to brown. The lighter the colour of the wood, the less tannin it contains. High tannin content found in oak, chestnut, acacia, nuc, mahoni and in most alien species with a more pronounced colouring. If a tannin wood splinter is left outside in the rain, it will stain and remain so after drying. It is possible to remove the stained surface by sanding, but on first contact with water it will happen again.

How tannins act and why they are unpleasant in wood

Tannins in wood can be troublesome, both because of their acidic nature and because they are soluble in water, and some of them even in organic solvents. Acidity of tannins disturbs the assembly of parts when wood comes into contact with iron or iron derivatives. Tannins react with iron to form blue or blue-grey dyes that stain wood. The strength of the wood is not affected, but the stain that appears can spoil the final appearance of the object. The reaction is favoured by the increased humidity of the wood. To avoid this phenomenon, it is best to use stainless steel products for joints.

The fact that are soluble in water is another feature that can lead to unpleasant effects. Applying baths or water-based varnishes, we allow the tannins to dissolve and migrate to the surface of the wood. The presence of tannins is not signalled by a specific appearance, so it is hard to know whether the wood will stain or not, even if it is of a species known to be tannin-rich. Therefore, if wood of a species known to have a high tannin content is to be finished, it is good to make sure that it will stay inside and not stain the finished surface.

Finishing tannin wood

In order for the finish not to be influenced by the tannin in the wood, we need to make sure that it stays inside, without the possibility of coming out. We can do this with insulators. These penetrate deep into the wood and react (harden), blocking the tannin inside. There are insulators for both exterior and interior wood. They can be water-based or solvent-based, single or two-component. Applied as a first coat and after drying, any finish can be used. Oils are not used because the insulator prevents them from penetrating deep into the wood.

One of the best insulation materials is however shellac. As tannin composition differs from wood to wood, ordinary insulators may not be strong enough for some of them. Shellac, however, can be a very good barrier between tannin wood and finish. It is so good that it can also be used for node isolation, locking in tannins and resin. The shellac coat, which can be applied with a brush, can be coated with solvent-based, water-based, alkyd or oil-based varnishes.

Tannins are really a problem when using white or light-coloured berries. In this case, the colour change will be noticeable, impossible to miss. When using dark colours or varnishes with a high yellow tint it is hard to notice the colour difference. Unless there are high concentrations in a particular place, so that it materialises as a stain, the phenomenon can go unnoticed.

Although problems can occur, we should not be afraid of working with tannin wood. The outward migration of tannin does not influence the physical and mechanical properties of the wood, it can only affect the appearance. As already mentioned, staining does not always occur, even if the wood has a high tannin content. If this were the case, very little work would be done with oak, which is far from the reality. But it's good to know why sometimes, even though we've done everything by the book, finishing problems occur that we've never encountered before.