Insufficient Film Thickness
Maximum dry film thickness (DFT) of coatings in contact with potable water surfaces is directed by extraction testing results to comply with the NSF Standard 61. Although that standard typically is concerned about excessive film thickness, many failures in water tanks can be attributed to insufficient dry film thickness. Since permeability of a coating is directly affected by film thickness, it stands to reason that relatively think coats of epoxy coatings can exhibit less than desirable permeability characteristics. When these coatings are put into water-immersion service, some molecular water will eventually get through the material and can reach either the substrate or a zinc-rich primer, which can result in significant failures of the tank lining.
These areas of low film thickness or insufficient film thickness typically will not be detected with holiday detection testing. Currently there is no acceptable nondestructive test or quality assurance mechanism that can point out insufficient film thickness of successive coats of paint. The only assurance we can have is by random checks of the dried film thickness using magnetic gauges. These random checks cannot be assurance of a consistent coating film, free of thin intermediate coats or topcoats.
Among the most common areas of coating failure and early corrosion are edges, sharp angles or radiuses of steel water storage tank interiors, both above and below the intended water level. These areas, if not addressed by stripe coating, will invariable result in low coating film thickness. Even the use of 100% solids epoxy coatings that meet “edge retentive” qualifications will often result in lower film thickness on edges due to the methods of application common in water storage tanks. Without actually observing the stripe coating or inspecting the tank prior to overcoating these stripe-coated areas, inspectors or others on water tank projects presently have no method to ensure that this crucial step is successfully completed according to specifications.
Virtually any coating can be made to fluoresce. Optically Active Additives (OAA) can be added to an existing coating very small amounts, similar to adding a color pigment to a coating. OAA will not normally have a detrimental effect on the coating or its desired performance.