Finishing defects - Lack of adhesion

Sometimes, in the factory, after the finishing process has been completed and the film has dried, you will notice that if the surface is knocked or something is dropped on it, a white mark appears. It looks like broken glass. You may think that the varnish is to blame, being too hard or too brittle. The appearance of the white mark actually indicates a lack of adhesion to the non-wood substrate. In that place, the varnish was easily loosened due to mechanical shock and air got between the varnish and the wood. The varnish film practically remains suspended in the air and cracks. Lack of adhesion can also occur between layers of applied material. In this case, too, it manifests itself by the appearance of a white streak following mechanical shock.

How to test poor grip without special machines

The simplest is coin test. Basically, you drag the edge of a coin over the lacquer-coated surface. If a fairly broad, white streak appears, it means that there is no adhesion, the layers have separated and air has penetrated.

Another way to check grip is grid test. In a less visible area, use a cutter to make a grid as in the "x and o" game. The resulting squares should be 1-3 cm square. Stick a piece of tape over the grid and tear it off. If more than 6 of the squares of varnish remain on the tape, the varnish has no adhesion.

Film breakdown occurs with solvent-based varnishes. In water-based varnishes, a different phenomenon occurs. The film formed after drying is the result of a three-dimensional polymerization that causes the molecules to be tightly bound together. If the film does not adhere to the substrate or to the undercoat, it is knocked off. A white streak still appears, but it does not break but can be peeled off like a skin.

What causes this and what to do to get a good grip on the substrate

The main cause of the defect is improper sanding or no sanding at all. Whether sanding the wood backing or sanding between layers, the process is equally important.

In the case of sanding of the wood support, the quality abrasive materials and replacing them when damaged is very important. Basically, adhesion is the result of the primer penetrating as well as possible into the wood and gripping it. The more properly sanded the wood, the rougher the wood will be to create the necessary roughness for good adhesion. This optimum roughness must be found. If it is too high, it will make it very difficult to sand the primer and reduce the quality of the finish, and too low a roughness will result in poor adhesion.

In general, final white sanding is 180-220 grit for furniture and 120-150 grit for windows. Abrasives should be changed when they are loaded with wood dust or after the period of use recommended by the manufacturer. Over-used abrasive tape becomes loaded with wood dust and the abrasive material is ground. The result is a finer-grained belt that no longer polishes the wood but smoothes it. In some places there can be very smooth and shiny areas, called mirrors, where adhesion is very low.

Too high viscosity in application is another cause of poor adhesion. Because it is very viscous, the primer cannot penetrate into the wood to set. In this case, thinning must be done according to the technical data sheets and application technologies, and the viscosity must be controlled with viscosity cups.

Adhesion between layers

The situation is the same when sanding between coats - roughness needs to be created so that the next coat can grip. There are exceptions. This is the case with nitrocellulose varnish, which does not need sanding for the next coat to adhere. The thinner in the topcoat softens the undercoat and the contact area blends.

This is not the case with polymerization (drying) curing varnishes such as polyurethane. For the next coat to adhere, the next coat must be applied immediately after sanding between coats. Polyurethane varnish coats take longer to dry in depth, but superficially dry in 2-3 hours. If the surface layer is sanded off, the softer part is reached and the adhesion is much better. If left for more than 3 hours after sanding, the topcoat dries superficially again and adhesion decreases. During the time until the surface is dry, the next coat can be applied without sanding, the so-called "wet-on-wet" method. It takes only 15-20 minutes for some of the thinner to evaporate.

Hydrodilutable varnish also needs to be sanded because its curing also takes place through polymerization. When water evaporates, the molecules come together and react with each other to form the varnish film. This is why you cannot attack the varnish film with its own solvent (water), as in nitrocellulose varnish.

As with the other finishing defects it is important to identify the causes and avoid them.

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.