Read Part 1 here.
Bleeding, blistering and metamerism can be frustrating to deal with in paint and coating formulation, as well as in consumer DIY projects. In this article, we look at each defect, as well as environmental factors and steps to take to avoid them.
Several types of bleeding can occur. In wood, brownish or tan discoloration on the paint surface can be caused by migration of tannins from the substrate through the topcoat of paint. This is typically caused by “staining woods,” such as cedar, redwood and mahogany, or over knots in many other wood species.
Tannin staining can occur with any kind of wood. Most tannins are extracted through the presence of water, while others are solvent-extractive. Application of latex coatings – especially ones with higher pH – directly to these woods may develop a stain on the finish coat when the paint is initially applied. If discoloration occurs days or months after application, water migration may cause the staining from the inside out. Staining substances will be carried from within the wood to the surface of the paint film.
In automotive coatings, pigments from the original finish may react with solvents in the coating, causing discoloration2. We also see nicotine deposits from tobacco smoke as another type of bleeding or staining.
Blistering is bubbles due to a loss of adhesion and separation of the paint film from the underlying substrate. There are many reasons that blistering can occur, including but not limited to:
- Poor surface preparation: painting over a contaminant to which the coating cannot adhere (such as mildew, dirt, or a severely chalky surface)
- Exposure of latex paint film to high humidity, moisture or rain, or frost shortly after the paint has dried, particularly with inadequate surface preparation and/or poor ventilation in areas such as kitchens, bathrooms and humid basements
- Moisture migration through the exterior walls seeps into the home
- Oil-based or alkyd paint applied over a damp or wet surface
- Painting in direct sunlight on a surface that is too warm can cause heat blistering
Although not necessarily a coating defect per se in the traditional sense, metamerism nonetheless is an issue. Metamerism is the phenomena of two colors that match under one set of conditions, but fail to match under a different set. Explained more technically, , a spectral power distribution describes the proportion of total light given off (emitted, transmitted, or reflected) by a color sample at each visible wavelength. It defines the complete information about the light coming from the sample.
However, the human eye contains only three color receptors (types of cone cells), which means that all colors are reduced to three sensory quantities; red, green and blue, called the tristimulus values. Metamerism occurs because each type of cone responds to the cumulative energy from a broad range of wavelengths, which can produce an equivalent receptor response and the same tristimulus values or color sensation.
There are three main types of metamerism:
- Illuminant metamerism occurs where different light sources cause the color to appear different.
- Observer metamerism is due to the individuality of observers and their physiology of eyes, etc.
- Geometric metamerism occurs when observing the same color and finish from different angles. We see an example of this in pearlescent finishes on automobiles.
This is the reason that light boxes used for color matching include numerous light sources. Most big box stores provide paint color cards/chips in a setting with multiple light sources, to better evaluate the color in different lighting.
Environmental and Application Contamination
Now that we’re looked at defect explanations and causes, we need to look at solutions to these problems. Avoiding the conditions that cause the defects can resolve each case. For additional information, paint companies including Sherwin-Williams and others provide many in-depth resolutions. The following provide insight into conditions that can create coatings defects, and are helpful to remember:
- Seasonal contamination by pollen produces a hydrophobic surface that is not easily wetted by water-based coatings. This can lead to film defects such as blistering, adhesion failures, fisheyes and crawling.
- Rain containing effluents from manufacturing, chemical industries and power stations is called acid rain. Some of the effluents may be acidic or alkaline in the presence of water (e.g. sulphur dioxide dissolves in water to create an acidic solution, whilst a mixture of cement dust and water is strongly alkaline.) Acid rain can deposit soluble salts on existing paint that can lead to similar failures cited above.
- When mildew and mold migrate through paint to the surface, they can form mildew on the topcoat. This can lead to many types of defects and failures.
Contamination from agricultural and horticultural sprays is seasonal/regional, and affected by bird/insect population. Organic etching is accelerated by intensified heat, such as from sunlight. Time and temperature dramatically increase concentration of acid. The damage is more visible on dark or darker colors due to heat absorption2.
- Although poor surface preparation or application under unfavorable conditions, such as in direct sunlight, before imminent rain or freezing, etc., cause many defects, the application method may also cause some defects.
- Extending the coverage rate (ft2/gal, m2/L) greater than the recommended level can result in poor coverage and substrate protection, thus leading to early failures or defects. This can occur particularly with paint application by spray, as well as when a paint is “thinned” with a solvent (water or organic).
- Generally, contaminated application equipment can cause localized failures and is not extensive through an entire project. For example, when a brush or roller contains some contaminants, such as oil in a waterborne paint, or water in an oil or alkyd paint, we see defects. Similarly, spray equipment can contain residual material from a previous usage that may not be compatible with the present coating.
- Using a foam brush can lead to microfoam or craters, especially when applying waterborne polyurethane and polyurethane/acrylic coatings to wood. This can be avoided by using the recommended equipment, or using a bristle brush immediately after foam brush application.
Many automotive coating defects are applicable to Industrial OEM applications as well:
- Static charge on a vehicle surface can attract dust.
- Dust and dirt from dry sanding, cloths, etc. can impact the surface.
- Unstrained paint results in particulate defects.
- Inadequate filtration of grease, oils, or other materials within compressed air can contaminate paint.
- Incorrect use of an activator or thinner used can lead to crawling, Benard cells, etc.
- Spray dust accumulated on spray booth surfaces/walls causes fisheyes and/or a disrupted finish.
- Particulates may foul the coating surface when using re-thinned 2K materials after the recommended potlife.
Other defects occur with specific application equipment or coatings. Some of these include spin (electronics or optics) coating application, and plasma vapor deposition for conformal coatings.
In summary, there are many reasons that coating defects can occur. Problems come about with poorly-formulated coatings, misuse of a coating in an unintended application, bad surface preparation or poor cure conditions. As with every coating, consider all conditions to achieve the intended performance.
- Sherwin Williams, Tannin Staining
- Axaltacs.com Paint Defects Manual [PDF]
- Colwell Colour [Metamerism PDF]
The views, opinions and technical analyses presented here are those of the author, and are not necessarily those of UL, ULProspector.com or Knowledge.ULProspector.com. While the editors of this site make every effort to verify the accuracy of its content, we assume no responsibility for errors made by the author, editorial staff or any other contributor. All content is subject to copyright and may not be reproduced without prior authorization from Prospector.