Why tiny pinprick-like holes appear in varnish or paint film

Pitting in the varnish or paint film sometimes occurs as a result of spraying finishing materials onto wood or MDF board. The cause can be air in the pores of the wood or inside the MDF board, drying too quickly with solvent trapping in the film, spraying water-based materials after solvent-based or vice versa without the gun and trails being thoroughly cleaned, oil escaping from the compressor, water from the compressor or in the spray gun trails. Even though each time it looks as if the varnish or paint film has been punctured with a thin needle, there are differences in the overall appearance of the defect depending on the cause of the defect. For example, in reference to the picture below, my opinion is that water from the compressor or hose got into the gun that was spraying solvent based materials. Why water appears in the compressor, what happens when droplets of this water end up in the spray gun, how oil from the compressor gets into the trails, and how we solve such problems, in the following.

On what grounds did I say that, in the case of the image above, the other defects are excluded

Air in the pores of wood. When the lacquer or paint is too viscous and the pores are large, air fails to escape from the pores and is trapped in the film. The defect occurs after the first coat of primer and after his sanding tiny pinpricks are visible in the film. These usually follow the pattern formed by the pores.

The air in the MDF board. MDF is a material made from wood fiber. The board is softer on the inside and denser on the surfaces, where there is also a layer of paraffin to protect it from moisture. After more intense milling or sanding, the looser, more absorbent part is reached. The air in this area tends to escape and as with pore air, can get trapped in the film. Pitting can be seen from place to place or spread all over the surface, depending on how loose the board is. Generally occurs in poor quality MDF. To look like in the picture above, the poor quality MDF would have to have been sanded more just in that area. In that case there would have been a very slight unevenness in that area which would have reflected the light differently.

Fast drying with solvent trapping in the film. It happens especially in the summer when it is very hot and the primer or lacquer hardens before the solvent comes out. The very small solvent droplets cause small pinhole-like holes to appear after sanding the primer or rough, unpleasant to the touch, rough surface on the varnish/paint. In this case too, the punctures are spread unevenly over the entire surface.

Spray water-based materials after solvent-based materials or vice versa. Solvent-based materials are not compatible with water-based materials and cannot be mixed. If a water-based varnish is sprayed after a solvent-based material (or vice versa) and the spray gun has not been thoroughly washed, solvent droplets may appear in the varnish which, when sprayed, are spread over the entire surface. The defect is visible as pin pricks spread unevenly over the entire surface. The same happens if the sprayed enclosure is not well ventilated before changing the material. Solvent vapors from the atmosphere fall onto the freshly painted surface and cause the pinholes. More information on such defects can be found at here.

Why water appears on spray trails and how to see the defect

Water in the compressor comes from air. Compressing the air to the desired pressure causes water vapor to condense. The higher the humidity in the atmosphere, the more water collects in the compressor. The accumulated water should be removed from time to time and filters should be fitted along the route. The filters must be as efficient as possible to retain the moisture. I have seen situations where the filter did not cope with the moisture in the air and water appeared in the small piece of hose between the filter and the gun.

When a drop of water gets into the spray gun, the pressure in the air turns it into thousands of tiny droplets, just as it does with varnish or paint. If the finishing materials are solvent-based, incompatibility will occur between them and water. When tiny droplets of water fall on the film a separation occurs, just as when a water droplet falls in oil or gasoline. Being micron sized, it is not seen as a crater but as a pinhole. The flaw appears clustered, many pinholes in one place, because the water is not continuously getting into the gun, but from time to time, causing the flaw to appear randomly.

Compressor oil causes the same type of defect

The same can happen if oil gets in the trails. The oil can also come from the compressor. In the spray gun, the compressed air acts on the oil droplet and turns it into many very small droplets that fall on the finished surface. The stings occur with both water-based and solvent-based products.

You can check for oil leaks by blowing air out of the gun, without varnish or paint, onto a white absorbent tissue or tissue. If there is oil, small oily droplets will immediately show on the surface of the paper.

How we solve the problem

First of all we need to make sure that there is very little risk of water getting on the paths. That means better filters and shorter routes. During periods of high humidity, the compressor needs to be checked more often and accumulated water removed.

The defect can be improved by adding an anti-crater additive to the varnish. It will not remove the water, but it will reduce the surface tension of the varnish and allow it to stretch better. The defect will not be so obvious and will be easier to repair. You can find such materials from professional wood varnish and paint suppliers.

If you already have pitted surfaces, first look for and remove the cause. To remedy, the surface should be sanded until the pitting is completely removed. Do not sand only superficially as subsequent layers will accentuate the defect. Sand the entire surface, not just locally. Then apply another coat of varnish or paint.

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.