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Testing times continued…

In our series on field tests and laboratory analysis, we consider solvent rub tests for the cure of epoxy resin and zinc silicate coatings.

The cure of two-part (chemically cured) coatings such as epoxies can be assessed using a solvent rub test. In this instance, methyl ethyl ketone (MEK) is used to confirm if the coating has cured. It is important to note however that the test will not determine the extent of the cure.

The test, which involves rubbing a cotton cloth soaked in solvent over the surface, will reveal if the coating is fully cured (which can affect applying further coatings) or not. Softening of the coating could occur, which may suggest an under-cured system. The basic procedure involves:

• Select a small area for the test (125 mm length)
• Measure the dry film thickness
• Use a folded cotton cloth saturated in the selected solvent
• Rub the cloth with the index finger held at 45 Degrees for a total of 25 rubs in each direction
• Inspect the coated area for finger hardness and visual changes (compared to the unrubbed areas)

ASTM D5402 Standard Practice for Solvent Resistance of Organic Coatings Using Solvent Rubs covers the detailed methodology that should be used to carry out this test.

A solvent rub test for zinc silicate coatings describes a technique for assessing the MEK resistance of ethyl silicate (inorganic) zinc-rich primers in either the laboratory, in the field or the fabrication shop. Ethyl silicate zinc-rich primers cure when the ethyl silicate binder reacts with moisture. As relative humidity and temperature alternates throughout the day so does the rate at which curing occurs. A minimum degree of cure is necessary prior to applying a top coat. The test procedure involves:

• Selecting a small area 150mm length x 25mm wide to test
• Use a folded cheese cloth saturated in the MEK solvent
• Rub the cloth with the index finger held at 45 Degrees for 50 double rubs
• Continue rubbing until the metal is exposed or the 50 double rubs have been completed
• Record the results in accordance with industry standards (ASTM D4752 Standard Test Method for Measuring MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rub)

For further information on field tests, refer to Fitz’s Atlas of Coating Surveys at