Finishing defects - Air bubbles. Pitting in the film

Often, after spraying finishing materials, air bubbles or pinholes can appear in the film. Although similar, these defects have different causes. I have chosen to present them together because they are often confused.

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The film has air bubbles included. Why it happens and how we can prevent or fix it

Air bubbles may occur in the varnish film (I will use this generic term for all film-forming materials) for various reasons, before or during application. If not enough time to deaerate, the temperature in the workspace is too high or the ventilation is too strong, drying occurs before the air escapes from the film. Let's take a look at what causes these bubbles and what solutions can be found to avoid or remedy the defect.

Mixing the material with air. During the stirring of the material so that it can be used, the varnish is mixed with a quantity of air. In order to prevent this air from remaining in the film, the mixture must be left for a while before application. Another way to remove air is to filter the material or use defoamers. Sometimes air mixing occurs in the pump itself. In this case the pump should be checked and recalibrated.

Too high temperature in the workspace. In the summer, when it is very hot and the temperature rises much higher than the recommended 20°C, the defect frequently occurs. In this case it is recommended to use slower thinners which delay the drying process and give the air time to escape from the film.

Other causes of the defect are too high air temperature or air velocity in the drying zone and too short de-aeration time. In this case, it is recommended to reduce the temperature and air speed in the drying zone and increase the flash-off time. In the case of automatic finishing lines, the flash-off time is increased by reducing the conveyor belt speed.

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Wood air. When wood is very dry, some of the water is replaced by air. The amount of air that escapes from the wood is greater and some of it may remain in the film. It is best to use wood with the recommended moisture content for wood used indoors of 8-12% (exception musical instruments) , and white furniture pieces should be kept in temperature and humidity controlled areas before finishing.

Incorrect spraying or high viscosity. Incorrect spraying, i.e. too small a spray distance, too high a viscosity or too low a varnish temperature, can also lead to bubbles. As with orange peel, the distance between the workpiece and the spray gun should be at least 25 cm and the pressure 2,8-3 at. for hand spraying.

Persistent foam from flow coating. When applying waterborne products by flow-coating, a very persistent foam is usually formed, which may remain embedded in the film and in the case of stains may lead to uneven coloration. The use of defoamers is recommended.

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Pins in the film

The appearance of the film in stings it looks like it's been pricked with a fine needle. The defect can be mistaken for air bubbles, especially after sanding between coats. Unlike bubbles, the punctures are not spread evenly, they appear clustered or here and there on the surface, they are also very visible immediately after spraying (before drying).

The defect occurs in the application of solvent-based lacquers when there is accumulated water (in compressors, hoses) in the spray paths and is much more frequent in very hot periods with high humidity. It is recommended to install filters on the compressor and on the air routes and to check the installations more often.

The same defect may also occur if waterborne products are sprayed in the same room as solvent-based products or if they are sprayed immediately after solvent-based products in the same enclosure. Fine solvent particles persist for some time in the atmosphere. If they are not removed by the exhaustion system, they settle on the film and cause stinging. All routes through which the solvent-based material has passed must be thoroughly washed with acetone and then with water (special thinner) before applying waterborne products.

I hope you find the information useful. If you have any questions or queries, please leave them below in the space provided. I will be sure to reply.

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