TIPS & TECHNIQUES
Peeling and Flaking Coating
When a coating loses adhesion to the substrate beneath, peeling and flaking can occur. Flaking is generally termed as the delamination of a brittle coating in small flakes, while peeling generally results in larger areas of the film coming away from the surface.
Peeling and flaking may be the result of insufficient surface preparation and surface contamination.
To ensure optimum adhesion of any coating to a substrate, or existing coating, it is important to ensure that the surface is in a clean, dry condition, free from dust, dirt, wax, grease and surface moisture. Always wear the correct eye protection approved to EN161 and respi-ratory equipment approved to EN149 FFP1S.
Where silicone sealants have been used and there is contamination to surfaces which need to be sprayed, adhesion of the coatings will be severely compromised. In many cases, it may not even be possible to wet out the surface with the coating, whereas in milder cases the coating may appear to spray out satisfactorily, but will later either pull away from the surface or will peel off quickly under exposure to the water.
Sanding a contaminated surface will only spread the silicone contamination, the only effective treatment is the use of a silicone remover or digester. In severe cases a second treatment may be necessary.
Coating Not Drying?
Failure of a coating to dry in the expected time can be for a number of reasons:
1) The drying times for most decorative coatings are usually based on an average temperature of 20 degree Celius. Drying times will naturally vary dependent on the actual temperature at the time of the application and may be lengthened considerably if the temperature is low. The higher the humidity, the longer the drying time as this will reduce the rate of evaporation of the solvent.
2) Poor air flow and ventilation will lengthen the drying time of any coating. It is quite possible for an applied coating to remain wet to the touch after 1 hour or more if it has been applied and then the area isolated with no air flow. With the introduction of adequate ventilation the drying process can begin.
3) An excessive thickness of coating will naturally take longer to dry than the recommended drying times stated for a product. The application of a thick coat instead of a suggested two-coat system is not recommended.
Cissing / Fish Eyeing
Cissing is also sometimes known as Fish-eying or when larger areas are exposed it is referred to as crawling.
Cissing occurs when the surface breaks within a wet paint film, exposing the underlying substrate. The paint is unable to wet-out the substrate and “pulls away” from the surface. This sometimes starts with a pinhole and may expand over some minutes to the eventual ciss.
Cissing is normally the result of surface contamination by either, moisture, grease, or silicone. It can be prevented in many cases by degreasing the substrate prior to coating using thinners or thinner on a clean, lint-free cloth. Dampen the cloth with the solvent and wipe over the surface, changing the face of the cloth frequently and allowing the solvent to evaporate before overcoating. In cases of silicone contamination, a silicone remover should be used.
Health Effects of Isocyanates
Isocyanates are highly reactive chemicals typically found in the hardener of two-part paints and primers. Isocyanates are present in two forms, monomer and prepolymer. The isocyanate monomer content is frequently indicated in Material Safety Data Sheets (MSDS), but this is only a small part of the total unreacted isocyanate present. Both forms
of unreacted isocyanate are a risk to health when they are released into the air during paint or primer spraying.
Breathing unreacted airborne isocyanate can cause coughing, chest tightness, fever, fatigue and sensitization. Many cases of isocyanate-related sensitization have occurred in Saskatchewan. Once a worker is sensitized, further exposure to even very small amounts of isocyanate will cause distressing asthma-like reactions. The reaction may occur immediately or several hours after exposure. One exposure to a high airborne concentration or several exposures to lower concentrations may result in sensitization. There is no proven method for predicting whether any particular person will become sensitized if exposed to Isocyanates.
Direct skin contact with isocyanates may cause rashes, blistering and reddening of the skin. Repeated skin contact may cause skin sensitization.
Eye exposure to airborne isocyanates can cause eye irritation and temporary blurred vision. Direct contact with the eye may cause damage to the cornea.
All spraying of isocyanate paints must be done in a commercial or engineered paint booth, or one that is acceptable to the local fire authority. The painter must wear an airsupplied respirator.
Although primers contain less isocyanate than paints, they must be applied in a paint booth unless there is a separate shop area which meets the following criteria:
1) The shop area must be separated from the rest of the shop by a barrier or screen.
2) The shop area must be equipped with an explosion proof ventilation system.
3) The barrier or screen and the ventilation system must cause air to move from adjoining areas into the priming area. This will prevent airborne isocyanates from entering areas where workers are not protected by air-supplied respirators.
4) If there is any risk of fire, the employer must consult with the local fire authority about the acceptability of the shop area.
Things that might create a fire hazard are:
1) the quantities of paint applied;
2) the method of application;
3) the location of the priming area; or,
4) the materials used in the construction of the priming area.
5) The person who applies the primer must wear an air-supplied respirator. Even short duration priming tasks release unacceptably high levels of airborne isocyanate. Low-pressure spray guns release less airborne paint, but levels are still high enough that only air-supplied respirators provide sufficient protection.
Paint Mixing and Cleaning
Paint mixing areas must have sufficient ventilation to prevent the build-up of airborne solvent. There is not enough unreacted isocyanate released during mixing to require the wearing of air-supplied respirators; however, a face shield should be worn where there is a risk of splashing paint into the eyes. Gloves should be worn to prevent skin contact. A cartridge-type respirator with organic vapor cartridges may be worn to reduce the amount of paint solvent breathed.
While cleaning spray guns by passing solvent through the gun under pressure, air-supplied respirators must be worn since unreacted iso cyanate may be released into the work environment.
Workers must always wear air-supplied respirators when spraying paints and primers containing isocyanates. Other types of respirators do not provide adequate protection for workers spraying isocyanate paints and primers.
Air-supplied respirators approved by the United States National Institute for Occupational Safety and Health (NIOSH) as type C constant flow or pressure demand are accepted by the Division for use with isocyanates. The Division will accept air-supplied respirators approved by testing agencies other than NIOSH in some situations. (Ask respirator
suppliers to provide a letter of acceptance issued from the Division for any respirators that do not carry NIOSH approval.)
Breathing air for the painter’s air-supplied respirator must be clean. A non-oil type electrical compressor specifically designed for providing breathing air is the best choice. Compressed shop air may be used if filters are provided to remove water, oil and he breakdown products of oil. The compressor must be kept in good repair. Overheating of the compressor must be prevented so that carbon monoxide will not be produced. Only clean air can be used to feed the compressor. The source of the air must be distant from vehicle exhaust to avoid unacceptable carbon monoxide levels in the air breathed by the painter.
The breathing air must be supplied to respirators with tight fitting face pieces at a rate of at least 115 liters per minute (4 cubic feet per minute), and of at least 170 liters minute (6cfm) to respirators with loose fitting face pieces. The air supply rate should not exceed 425 liters per minute (15cfm).
The airline feeding the spray gun may be split to also feed the supplied-air respirator as long as the air is filtered in the prescribed manner and supplied at the prescribed flow rate.
Full coveralls are required when spraying isocyanate paints and primers. Synthetic rubber or PVC gloves must be worn. These materials prevent isocyanates from reaching the skin.