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July 10 - July 16, 2016

What causes bubbling or cratering of epoxy intermediate coat when applied over epoxy zinc-rich primer?

Selected Answers

From Dwight Weldon of Weldon Labortories on August 4, 2016:
First of all, it should be recognized that bubbling and cratering are two different things. In fact, visually, they are virtually the opposite of one another. It is well known that coatings, such as the epoxy mentioned in this question, can pinhole when applied over inorganic zinc rich primers. This is because inorganic zinc primers, with their very high loading of zinc dust, tend to be somewhat porous. This allows both air and perhaps moisture to be trapped in the pores. When a relatively viscous, high solids coating such as an epoxy is applied over such a surface, it will flow into the porosity of the primer and try to displace the air and/or moisture. The most common problem associated with this is pinholes in the intermediate coat, although perhaps under certain conditions of temperature and humidity, it might be possible for bubbles to form. This is usually avoided by applying a thin mist coat of the intermediate (usually by thinning the epoxy). Not only can this thin coat of low viscosity material more effectively displace the air/moisture in the zinc primer, but because it is of low thickness, the air/moisture can more readily escape through it. Furthermore, being of low viscosity, it can flow out and “heal” any pinholes which might have been formed by the escaping air/moisture. While the above phenomenon is somewhat common for inorganic zinc-rich primers, it is much less so for epoxy zinc rich primers. This is because epoxy zinc-rich primers tend to contain significantly less zinc dust than do the inorganic variety, and hence are less porous. On rare occasions, it is possible for solvent trapped in a lower coat to cause bubbles or blisters in an upper coat, if the upper coat has been applied too soon. This is very dependent on the chemistry of the two coatings, the application parameters, and the environmental conditions. However, under most circumstances, the solvent from the lower coat will very slowly migrate out of the coating system over time, without causing bubbles. Part of the reader’s question involved cratering. If actual craters are occurring, as opposed to bubbles or voids, this is almost always due to some form of low surface tension contamination on the surface of the primer. For one coating to wet and flow over another, the surface tension of the applied coating must be equal to or less than the surface tension of the lower coat. Minute amounts of low surface tension contamination (oils and silicones come to mind) on the surface of the primer will prevent this, leading to craters.

From Charles S. Brown of Greenman-Pedersen, Inc on July 29, 2016:
The main cause is solvent entrapment from applying the intermediate coat before the zinc primer has cured. One of the most critical aspects of applying any coating is to review and understand all the information presented on the paint manufacturer’s product data sheet prior to application. This will help to eliminate these kinds of problems.

From Thomas Elsbernd of M1 Support Services on July 21, 2016:
The most likely cause is solvent popping due to trapped solvent under the partially cured paint film. This becomes prevalent when applying a thick layer of product that promotes a skim to form on surface, entrapping solvent pockets underneath. As the pressure builds, the solvent escapes producing a ballooning of the surface. As the solvent dries, it leaves a weak film that is easily crushed by touching. It can also cause craters if the solvent pressure is enough to clear the surface tension, punching holes in the surface of the coating.

The cause is most often solvent entrapped in the primer film. During the curing process, solvent is evaporating through the paint film, which is the main cause of blistering and bubbling. This defect can also occur if the surface is contaminated.

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Tagged categories: Asia Pacific; Coating failure; EMEA (Europe, Middle East and Africa); Epoxy; Latin America; North America; Paint defects; Zinc; Zinc-rich (inorganic); Zinc-rich (organic)

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