Zinc Rich Paint for Galvanized Steel Touch-Up or Repair

The method to repair hot-dip
galvanized coatings in the plant or in the field is contained in ASTM A780. The
specification includes three acceptable materials for touch-up and repair of hot-dip galvanized steel: zinc based solders, zinc rich paints, and zinc spray
metallizing. This video will review touch up with
zinc rich paints. Zinc rich paint is either brushed or sprayed onto a clean,
dry steel surface. The concentration of zinc dust in zinc rich paints used for
touch-up is recommended to be either 65 to 69%, or greater than 92%
metallic zinc by weight in the dry film. Both organic and inorganic zinc paints
can be used. Zinc rich paint is the most common of the specification repair
materials to apply in the shop and in the field. The area to be repaired should
be clean, dry, and free of oil, grease, pre-existing paint, and corrosion
products. If welds are in the area, ensure any weld flux or spatter residue is also
removed by grinding or chipping it away. Where field service conditions include
immersion, blast clean the surface to SSPC/SP10 NACE number 2 near white
metal. To provide a smooth repair coating, the blast cleaning should extend into
the surrounding galvanized coating. For less aggressive field conditions, power
tool cleaning to bare metal may be performed to SSPC/SP 11 as a minimum. When blasting or power tool cleaning is not practical,
hand tools may be used in accordance with SSPC SP2. Apply zinc rich paints
as soon as possible after surface preparation to avoid the development of
any oxide. Follow the paint manufacturer’s instructions and be aware
of the recommended temperature and humidity conditions, curing time, and
paint thickness per application. Spray or brush the zinc paint onto the surface
with horizontal passes per the manufacturer’s
recommendations. Once the repair coating has cured, the product can be inspected
and the thickness verified by electromagnetic, magnetic, or eddy current
measurements. The final coating thickness for areas repaired with the zinc rich
paint should be 50% more than the galvanized coating thickness requirement,
as specified in ASTM A123, A153, and A767, but not more than 4.0 mils, 100
microns. Multiple applications of zinc rich paint may be necessary to achieve
coating thickness repair requirements. Follow the manufacturer’s curing
instructions between applications.

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