Determining the Proper Cure of Inorganic Zinc Primers

Ethyl Silicate Inorganic zinc primers commonly found in the United States coating market include Dimetcote 9, Interzinc 22, and Carbozinc 11. These primers cure through a chemical reaction with moisture in the air. Normally, inorganic zinc (IOZ) primers are later coated with an epoxy which provides an effective barrier coat to protect the IOZ and the steel from the environment. Because epoxy provides such an effective barrier coat, it is extremely important that the IOZ be cured prior to application of the next coat. If IOZ curing does not take place prior to application of the next coat, the complete curing will never occur. For this reason, it is important to have a reliable test method in place to determine an acceptable level of IOZ cure. With any coating, it is critical that the coating manufacturer’s written recommendations are closely followed.

Finding the Right Level of Inorganic Zinc Cure

Two methods are commonly utilized. As noted earlier, check with our team or the manufacturer for product-specific recommendations. Remember, if you aren’t using an ethyl silicate inorganic zinc primer, these methods won’t apply. Make sure you know what generic product type you are using before using these test methods.

Two Test Methods to Try Before Applying a Second Coat

The first test method is the coin rub test. When the edge of a coin is pulled across the surface of properly cured IOZ, a silver scar will be observed on the surface of the IOZ. Any other result would indicate an incomplete cure that is not ready for the next coat. Unacceptable results may include a depression in the IOZ or the accumulation of IOZ on the coin. Just look for the silver scar and you’ll know the IOZ is cured. It is best to ask the coating manufacturer if this method applies to your specific coating.

The second test is the MEK Solvent Rub test. This method utilizes a cloth over the test person’s finger, which is dipped in Methyl Ethyl Ketone. It is then rubbed on the IOZ with 50 double rubs.  A double rub means forward and back, utilizing moderate pressure. After fifty double rubs, the cloth should show no IOZ residue or only a slight residue. The most commonly used test method is ASTM D4752, Standard Test Method for Measuring MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rub. This test method provides detailed instructions of the procedure, as well as a grading scale to record the level of cure based on the observations of the cloth and the coating after performing fifty double rubs. Remember to consult the manufacturer’s written instructions for specific requirements when using this test method to determine acceptable cure.

Reasons to Try IOZ Primer Cure Tests

Many applicators simply wait for the manufacturer’s recommended cure time to pass prior to recoating. However, their instructions are missing one very important consideration: the HUMIDITY level. IOZ’s are one coating category that requires a minimum humidity level, and if the recommended humidity level isn’t present during curing, the appropriate cure won’t be achieved. This occurs frequently in the winter months of low humidity and in dry desert regions any time of the year. While most types of coatings (acrylics, alkyds, epoxies, and urethanes) cure more quickly with low humidity, ethyl silicate inorganic zincs cure more quickly with high humidity.

How to Speed Up Curing of Inorganic Zinc

So, what can you do if you need to speed up the curing of an IOZ? Most coating manufacturers recommend an initial cure time followed by a water mist for several hours. Consult Strand’s Industrial Coatings for specific recommendations for your coating.

More information on this topic can be found in:

SSPC-PS Guide 12.00  

Coating System Guide 12.00

Guide to Zinc-Rich Coating Systems

ASTM D 4752 Standard Test Method for Measuring MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rub