Chapter 5 - Surface Quality Aspects

Abstract

This chapter focuses on the various factors that affect the performance of protective coatings of paint and related products applied to steel surface prior to immediately prior to painting. The principal factors to influence this performance are the presence of rust and mill scale; the presence of surface contaminants, including salts, dust, oil and greases; and the surface profile. Numerous standards have been issued to define these factors, and testing methods are available to quantify them. The ability to adhere to the substrate throughout the desired life of the coatings is one of the basic requirements of a surface coating, second only to the initial need to wet the substrate. Chloride content on the substrate significantly influences the performance of coating systems. Rust development under the paint film and osmotic blistering are commonly observed at an early stage in paint coatings applied over contaminated steel substrates. Thresholds for chlorides and sulphates also depend on dry film thickness (DFT) of the applied paints. Embedded grit may act as separators between substrate and coating system, similar to dust. If abrasive particles are notably contaminated with salts they may even cause rusting and blistering.