Finishing Defects - Staining, staining, fading, colour migration

The color applied to a wooden 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 staining depends on several factors which, if not respected, will result in staining defects, i.e. stains, discoloration, discolorations and color migration.

Stains due to different absorption

Stains most often occur due to improper sanding. In this case, in certain places, the absorption in the wood is increased or decreased, resulting in darker or lighter colored stains. Here's how to sand to avoid defects here.

But there are also situations where, no matter how well the wood has been sanded, stains still appear. This is the case with fast-growing species, where the wood is softer, looser and more absorbent and absorbs more color (poplar, willow, anise). In this case it is better to use uniforming staining solutions or special resin-containing stains that reduce absorption into the wood. Even staining can also be done after priming, but this method is not effective if the stains are very visible.

A special case of increased absorbency are the corners or stru stru strujite areas, where wood fiber ends appear to absorb more easily and quickly. The use of insulators that block absorption in the wood fiber is recommended.

Putty or glue stains can alter absorption leading to staining

In the case of veneered panels, stains may occur in areas of glue penetration due to improper adhesive dosing or low-quality veneer with fiber pull-outs. In the area where the adhesive has penetrated, absorption is blocked and a light-colored stain appears.

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

Stains also occur 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 everything down and start the process over.

Chemically bleached wood changes the color of polyurethane primer

Primer coloring problems 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 color of the primer to a deep yellow. If the wood has been discolored or chemically bleached, the finish should be done with acrylic hardeners. They also have the advantage that they do not change the base color at all and are recommended for protecting special effects, very light-colored finishes or light-colored natural woods.

Stains due to wood tannin

There may also be problems with coloring when waterborne finishes are applied to tanned woods such as oak, acacia or exotic woods. The phenomenon is even more visible if white or light-colored pigmented products are applied. For example, if a white waterborne primer is applied over acacia, almost immediately the white color will start to turn green. To avoid this, the use of insulators is recommended.

Wood tannins can also cause problems with bleaching coloring solutions. Here again, the solution is to use insulators before applying the stain.

Colour change due to light

Discoloration can also occur due to exposure to UV radiation. This is the case with staining solutions used for staining outdoor products, those in display windows or in areas highly exposed to direct light, or those that will pass under UV lamps if such a finish is applied. In all these cases, UV and direct light resistant products should be used. Specially formulated exterior materials must be used. If interior-formulated dyes are used outdoors and the finished furniture sits in the sun, in less than a year that color, no matter what it is, will fade to gray.

For products used indoors but in brightly lit areas or in shop windows, use colorants that are resistant to direct light and coat with UV-absorbing materials.

Colour migration

Sometimes, when you apply waterborne primer over a colored surface, you can see how the color starts to migrate in the primer. The phenomenon is also called bleeding. It usually happens with universal colorants, which are soluble in both water and organic solvents. When a water soluble primer is applied over it, the water it contains solubilizes the dye and the dye begins to migrate into the primer, staining it. The same can happen if a solvent-based primer, also solvent-based, containing a more volatile heavy diluent, is applied over the solvent-based stain. The solution is to use a paste that is insoluble in the product to be applied. More special gins may also be used which, once fixed to the wood fiber, do not dissolve in any thinner.

My recommendation is that you should always provide the supplier of finishing products with all the information about the nature of the substrate, the treatments it has undergone, the finishing technology used, the destination of the finished product and other such information that will help them to recommend the most suitable finishing products and solutions. 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 here here.

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