Finishing Defects - Staining, staining, fading, colour migration

The colour applied to a wood surface interacts with both the wood and the subsequent finish coat. Trace interactions also occur between the primer and the wood substrate. The quality of the stain depends on several factors which, if not respected, will result in the appearance of stain defects, i.e. staining, staining, discolouration and colour migration.

Stains due to different absorption

Stains occur most often due to improper sanding. In this case, in some places, the absorption in the wood is increased or decreased, resulting in darker or lighter stains. How to sand to avoid defects can be found in here.

But there are also situations where, no matter how well the wood is sanded, stains still appear. This is the case with fast-growing species, where the wood is softer, softer and more brittle and absorbs more colour (poplar, willow, aniseed). In this case, it is best to use even staining solutions or special resin-containing stains that reduce absorption into the wood. Even staining can also be done after the application of primer, but this method does not work if the stains are very visible.

A special case of increased absorption are the edges or grooved areas, where the ends of wood fibre appear to absorb more easily and quickly. It is recommended to use insulators that block absorption in the wood fibre.

Putty or glue stains can alter absorption leading to staining

In the case of veneered panels, stains can occur in areas where there are glue penetrations due to incorrect adhesive dosage or low quality veneer with fibre pull-outs. In the area where glue has penetrated, absorption is blocked and a light-coloured stain appears.

The solution that was used a lot in the past was to stain the adhesive. It was very restrictive, because afterwards you could only stain in the colour of the adhesive or in a very dark colour. That's why it is recommended to use a staining solution that has a rapidly volatile thinner, such as acetone, as a solvent and to apply it by spraying. In this way, the colour stays on the surface, does not penetrate the wood and settles over the area with glue penetration. However, glue penetration should be avoided because, even if the problem of staining is solved, the problem of adhesion to the substrate of subsequent coats remains.

Stains also appear in areas where the wood has been repaired with putty and the stain is not compatible with that putty. Compatibility between the two products should be tested before using them because, if they have been used, the only way to fix the defect is to sand the whole thing and start the process again.

Chemically bleached wood changes the colour of polyurethane primer

Problems with staining of the primer occur if a polyurethane primer is applied over chemically bleached wood. The catalyst is very sensitive to the substances with which the wood is bleached and turns the colour of the primer to a deep yellow. If the wood has been discoloured or chemically bleached, finishing should be done with hardening acrylics. They also have the advantage that they do not change the base colour at all and are recommended for protecting special effects, very light coloured finishes or light coloured natural wood.

Stains due to wood tannin

There may also be problems of coloring when finishing tannin-treated wood such as oak, acacia or exotic woods with water-based products. The phenomenon is even more visible if white or light pigmented products are applied. For example, if a white water-based primer is applied to acacia, almost immediately the white colour will start to turn green. To avoid this, the use of insulators is recommended.

Also due to the tannins in the wood, problems with bleaching of staining solutions. In this case, the solution is also to use insulators before applying the bath.

Colour change due to light

Discolouration can also occur due to exposure to UV radiation. This is the case for staining solutions used for staining outdoor products, those that sit in shop windows or in areas that are highly exposed to direct light, or those that will pass under UV lamps if such a finish is applied. In all these cases, products that are resistant to UV radiation and direct light should be used. Specially formulated materials should be used outdoors. If formulated indoor stains are used outdoors and the finished furniture sits in the sun, in less than a year that colour, no matter what it is, will fade and turn grey.

For products used indoors but in brightly lit areas or in shop windows, dyes resistant to direct light should be used and coated with UV-absorbing materials.

Colour migration

Sometimes, when applying water-based primer over a coloured surface, it is observed how colour starts to migratein the primer. The phenomenon is also called bleeding. This is usually the case with universal dyes, which are soluble in both water and organic solvents. When a water soluble primer is applied over, the water contained solubilises the dye and it starts to migrate into the primer, staining it. The same can happen if a solvent-based primer containing a more volatile heavy-duty thinner is applied over the solvent-based stain. The solution is to use a stain that is insoluble in the product to be applied. More special stains can also be used which, once fixed to the wood fibre, do not dissolve in any thinner.

My recommendation is that you always provide your finishing supplier with all the information about the nature of the substrate, the treatments it has undergone, the technology used for finishing, the destination of the finished product and other such information that helps them to recommend the most suitable finishing products and solutions. In this way you avoid defects that are difficult, if not impossible, to remove at no extra cost.

The full series of finishing defects can be found at here.

I hope you find the above information useful. As usual, additions are welcome. And if you have any questions or queries, please leave them in the space below. I'm sure I'll reply.