KHD-327
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-15-11.MAINTENANCE PAINTINGWhen to repaint:Even if a steel structure has been painted with the greatest care, the paint film will gradually deteriorate over time.Rust will eventually appear where its thickness is insufficient or where there are discontinuities such as pin holes.On structures which were poorly treated, blisters, peelings and other defects in the paint film will occur earlier than expected.Once heavy rust delevops, repainting becomes costly because the entire surface must be retreated.Chemical surface treatment, such as picking, can not be applied on fabricated steel structures. And blasting work will often be inadequate, too, in view of dust pollution.Due to the reasons above, cleaning using a power tool or hand tool is the only means of surface preparation in some cases even though an unsatisfactory result can be anticipated. On the other hand, if maintenance painting is done at the proper time, repair will be simpler and costs will be greatly reduced.Paint film deterioration differs depending on the kind of paint.Conventional paints such as oil-based paint, alkyd paint or chlorinated rubber paint loses its gloss as the surface layer decomposes. When decomposed substances accumulate on the surface, it is called "chalking." Water, oxygen and other corrosion-accelerating substances penetrate the film and react with the steel substrate.The paint film loses its flexibility due to the oxidation, decomposition and evaporation of some components.As a result, blisters, cracks and other defects besides chalking also appear on the paint film.To prevent the film defects mentioned above, periodic repainting is recommended for steel structures that have been coated with conventional paint. It is most economical to repaint over old paint film which has a rust ratio between 0.2% and 0.5%. The European Scale of Rusting for Anticorrosive Paints, SSPC Vis 2 and ASTM, are shown. They are widely used to rate rust conditions.The deterioration of protective coatings begins only where the paint has been inadequately applied. From this point of view, steel structures protected with protective coatings must be inspected regularly and all defects must be immediately repaired. It can be said that thorough inspection for protective coatings at the time of the construction completion of the structure is essential.The timing and mode of repainting depends on the condition of the corroded parts. Paint manufacturers should also be consulted before repainting because many types of synthetic resin are used as binders in the formulas of protective coatings. The compatibility of old and new paint must be considered.Table 7 and Fig. 8 show how repairs should be made in accordance with the condition of the old paint film. Table 8 shows the compatibility of the new paint used for repair or overcoating.

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