Let’s Talk About Cure Testing of Inorganic Zinc Rich Primers

KTA’s Certified Coating Inspector Forum Volume 2, Issue No. 6 – June 2023

William Corbett, COO

AMPP Senior Certified Coating Inspector & Certified Protective Coating Specialist

KTA’s Certified Coating Inspector Forum is designed to provide professional development/continuing education on standards, inspection practices, new instruments, and other topics to help keep certified AMPP and FROSIO coating inspectors current. It represents the views of the author and KTA-Tator, Inc. It may or may not represent the views of AMPP: The Association for Materials Protection & Performance, even though SSPC, NACE, and AMPP standards are frequently referenced in the content.


Zinc-rich primers on new steel in the shop today include both organic (epoxy) type (OZ) as well as inorganic (ethyl silicate) type (IOZ). Although IOZ was more common in the 1970’s, 1980’s, and 1990’s, it is still used today. This issue of Certified Coating Inspector Forum serves as a reminder that IOZ primers need special attention to verify they are not just dry, but are indeed adequately cured, prior to overcoating.

High temperatures accelerate the curing of many convertible coatings like epoxy and polyurethane, but if the relative humidity is too high, curing of these materials can be compromised.  But for ethyl silicate IOZ primers, while high temperatures speed up the curing, moisture is required and is the most important environmental factor in the curing process. IOZ primers release solvents (primarily ethyl alcohol) during the drying phase and require humidity (moisture) to cure. While adequate humidity is typically not a problem in the summer, it can become an issue in the winter, especially for work in shops located in colder climates where the air in heated shops typically has a very low moisture content. The solvents evaporate from the applied primer because of the heat, which can result in a coating that is dry-to-touch in 30 minutes or less. But without sufficient moisture in the air the IOZ can remain inadequately cured for topcoating for days. Topcoating too soon prevents the necessary moisture from reaching the zinc primer to cure it. Contractive curing stresses from a solvent-borne overcoat (e.g., polyamide epoxy) applied to an uncured IOZ can cause a cohesive split in the IOZ primer, leaving some of it on the steel and some on the back of the disbonded epoxy. In some cases, the uncured primer is also pulled completely from the steel surface. Unfortunately, these issues may not appear until the coated steel is being handled and erected at the project site (see photo).  

So, the ability to differentiate between drying and curing of IOZ primers prior to topcoating is a key point. There are three tests that can be performed on the IOZ to verify it is sufficiently cured, including a solvent rub test, a pencil hardness test, and a coin rub test. Each of these will be briefly described. But before doing so, I want to spend a little more time explaining the need to “verify cure.” Coating manufacturer’s Product Data Sheets will often indicate a minimum relative humidity and cure time for their IOZ product. But a certified coating inspector should never assume that because these minimum conditions have been met that the product is indeed cured. Instead, it is important to verify. Each of these tests takes only a few minutes to perform but could save thousands of dollars in rework, not to mention potential liquidated damages for project delays.

Solvent Rub: ASTM D4752, Standard Practice for Measuring the MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rub was first published in 1987 and is specific to IOZ primers. Briefly, a cotton cloth is saturated with methyl ethyl ketone (MEK) and using moderate finger pressure the cloth is moved back and forth (a 6” path) against the primed surface. One stroke forward followed by a return on the same path is called a double rub. The resistance to the MEK rub is rated from 0 to 5 according to the table in the standard (a rating of 5 indicates no zinc transfer to the cloth after 50 double rubs, while a rating of 0 indicates the zinc primer was removed to expose the underlying steel in 50 double rubs or less). An adjacent area is tested using a dry cloth to assess the effect of burnishing without MEK (control). A rating of 5 typically indicates the IOZ can be successfully overcoated, but the acceptance value must be confirmed with the coating manufacturer. Note that other convertible coatings (e.g., epoxy) can be tested for solvent resistance according to ASTM D5402, Standard Practice for Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs, which employs a similar process, but doesn’t require the use of MEK (other solvents can be used as determined by the manufacturer).

Pencil Hardness: ASTM D3363, Standard Test Method for Film Hardness by the Pencil Test is referenced by at least one major coating manufacturer to test the cure of their IOZ. According to their Product Data Sheets, a minimum 2H Pencil Hardness is required prior to recoating, which means a 3H pencil will gouge the applied film, but one pencil softer (2H) will not gouge the film. Pencil hardness testing should only be used when required by the project specification or product data sheet.

Coin Rub: While there is no ASTM test procedure for the coin rub test, an experienced user can get a sense of whether the IOZ primer is cured. For this test, a quarter is held at approximately 70 to 80 degrees to the surface (perpendicular is 90 degrees) and the edge is scraped along the surface of the primer. If the surface burnishes (becomes shiny) the IOZ is likely cured sufficiently, whereas if powder is generated by the scraping action the applied film remains uncured.

While knowledge of how to perform these tests is important for the Quality Assurance (QA) Inspector (representing the facility owner), it is the role of the Quality Control (QC) Inspector to perform them, since they work for the shop that is contracted to prepare and coat the steel, and they are responsible to control the quality of the work. The QA inspector can verify the tests are being performed and documented correctly, and even perform their own confirmatory testing as necessary.

While the means and methods of changing the conditions to achieve the proper cure are clearly up to the shop/contractor, misting the applied primer with potable water two hours or so after application or increasing the humidity in the air has been used. Other processes may be recommended by the coating manufacturer. The tests described above can be used to determine the effectiveness of these methods.


This issue of the KTA Certified Coating Inspector Forum explored various methods of testing an IOZ primer for cure prior to overcoating. Inorganic zinc-rich primers are used on newly fabricated steel in the shop. These primers frequently dry very quickly but may not cure for days unless there is sufficient airborne moisture available to react with the ethyl silicate binder. Confusing drying with curing can result in catastrophic failure. A knowledgeable inspector will verify cure even if prevailing conditions of temperature and humidity conform to the manufacturer’s requirements.

4 thoughts on “Let’s Talk About Cure Testing of Inorganic Zinc Rich Primers”

  1. Excelente el artículo, por ello es muy importante la evaluación de las condiciones ambientales en el lugar de trabajo, con ello, nos daremos una idea clara y precisa, para enfrentar el curado de IOZ y complementando claro está, con las pruebas de campo señalados en el artículo.

  2. Gordon Kuljian

    As always, this is a concise, informative article highlighting the critical features of evaluating IOZ cure.

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