Surface preparation

The nature of the surface to which coatings are applied has a major effect on their performance. Generally, the better the surface preparation, the better the long-term performance of the coating. Some manufacturers produce coatings which are tolerant of poorly prepared surfaces in order to facilitate maintenance and repainting. In all cases, the method of preparation should be compatible with the coating system and the manufacturers' data sheets should be consulted.

For high-quality coatings, it is essential to remove all the contaminants, rust and mill-scale, which forms when the hot surface of the rolled steel reacts with air to form an oxide. For interior environments, where a low level of protection is required, the surface should be clean and free of loose rust and mill-scale, but a high level of preparation is not generally required, depending on the system chosen.

Surfaces embedded in concrete should be free of mill-scale but may otherwise be untreated, as the alkalinity of the concrete passivates the steel. Treatment on adjacent areas should be returned for at least 25 mm within the concrete. The cover to the embedded

steel should be in accordance with the requirements of BS 8110.

Various methods are adopted for cleaning steelwork, but the only effective methods for removing all mill-scale and surface containments are blast-cleaning and/or pickling (immersing the steel in acid). Other methods, such as flame cleaning and wire brushing, may be useful for maintenance or preparing steel for use in mild conditions, but they will not remove all the rust and mill-scale.

The method and quality of the surface may be specified according to BS 7079,50 some of whose parts are replaced by BS EN 8501, 8503

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and 8504. ' Conventional specifications may ask for shot blast to standard SA 2.5, or wire brush to standard St 2. Both grit and shot-blasting are possible, but shot-blasting is preferred for smooth high-quality paint coatings, and grit-blasting is preferred for galvanizing and some primers. Galvanized surfaces need not necessarily be gritblasted but a thinner coating will result. Usually the manufacturer will specify the type of surface required for the application of a particular coating. Where the surface is blast-cleaned, any delay between this cleaning and the application of the first coating should not exceed four hours in order to prevent further rusting and contamination.

When site painting is adopted, it is important that the steelwork is cleaned before paint application. This may require washing of the steel with a suitable detergent to remove contamination that has occurred during transportation and erection.

For connections exposed to severe environments, it may be necessary to blast-clean the connections before applying the protective system, but this is time consuming and expensive. Alternatives include the use of galvanized bolts, de-greased after tightening, followed by etch priming and painting to the same specification as the adjacent surfaces.

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