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December 17 - December 23, 2012

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

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Selected Answers

From sasha bacic of none on December 18, 2012:
    The zinc-rich primer is a porous coat in which the air is entrapped. To avoid bubbling, there are two solutions: overcoating with a specifically formulated, so-called "sealing" coat before application of the subsequent coat; and using the so-called technique, "full-over-the-mist," which consists of diluting the coating up to 60% or more, then applying it in the form of a mist onto the zinc primer. The highly diluted paint will penetrate into the porosities of the primer and seal them. The next step is to apply the non-diluted paint immediately after the mist coat to ensure perfect adhesion. Of these two methods, the applicator may select the one with which he is more comfortable (depends of the experience of the applicator) and which is acceptable to the client. Comparative testing is recommended to assert which method to select. Contractor and client shall sit down and agree on testing parameters and evaluation criteria. It is far better to discuss before than after.

From Jorge Lizarraga of International Paint on December 17, 2012:
     Bubbling is caused by the air trapped among the zinc particles. To avoid this effect, it is necessary to apply a very diluted first coat in order to displace the air. Then, coat it as normally.

From Kiran Pawar of Berger Paint Bahrain on December 17, 2012:
Porosity present in the zinc-rich primers is responsible for the generation of bubbling or blistering when topcoated with thick coatings. In order to avoid this situation, apply a thin coat as a mist coat before going for thick coat.

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