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.
