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Byline: A. J. Vazquez Vaamonde, J. J. De Damborenea Gonzalez Changes in steel making, such as continuous casting, have been of great influence in the behaviour of steel during hot-dip galvanising. The extent of these changes, in terms of world production, and the interest in them are considered. A review of this behaviour, the so-called 'Sandelin effect', which results in the formation of a higher-thickness coating and a modification of its structure, is given, taking into account the points of view of the steel maker, the galvaniser and the user. Some basic aspects of the process are presented, with reference to the influence of the different parameters involved in the reaction between steel and zinc. Practical solutions to this problem - modification of the composition and temperature of the zinc bath, modification of the speed of withdrawal, modification of the pre-treatment given to the surface, etc. - are discussed. Some positive aspects of this effect, such as higher resistance to corrosion in different weathering conditions, are also considered.