RENOVATING DAMAGED COATING
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Small areas of the coating may be damaged by operations such as cutting
or welding after galvanizing and, although a galvanized coating has
excellent resistance to rough treatment, small areas of damage may
occasionally occur in transport and erection. Due to the sacrificial
action of zinc, small localised flaws do not reduce protection.
Nevertheless, it is often aesthetically desirable to renew the coating
even over such small areas.
Adequate corrosion resistance will be achieved at any damaged area if a
weight of zinc is deposited equivalent to the weight of the undamaged
coating. The following techniques are acceptable according to BS EN ISO
1461:
- Thoroughly wire brush the affected area and apply sufficient
coats of zinc rich paint (by brush or aerosol spray) to give a coating
thickness in accordance with supply standard e.g.EN ISO 1461.
- Thoroughly wire brush the affected area, heat it with a blow torch to
300°C and apply a special zinc alloy rod or powder to obtain required
repair coating thickness. This may, however, be difficult or too
time-consuming to carry out on erected steelwork, or on awkward, or
inaccessible areas. Care should be taken in applying heat by a
blowtorch as excessive heat may damage the surrounding galvanized
coating, especially on the heavier coatings resulting from reactive
steels.
- Grit
blast the affected area and thermal zinc spray. A 100µm thermally
sprayed zinc coating confers corrosion protection equivalent to an 85µm
galvanized coating.
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Zinc rich paint is much the simplest to apply, especially on site.
Thermal zinc spraying is usually only economic when applied in the
workshop.
BS EN ISO 1461 requires that the coating thickness on renovated areas
shall normally be 30µm more than the local coating thickness
requirement for the galvanized coating. An exception to this may be
when the coating is to be overcoated and the renovated area then
requires an equivalent thickness.
Purchasers and applicators of subsequent coatings should ensure that
the applied system is compatible with the method and materials used for
renovation.
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