When hot dip galvanized coatings are damaged by cutting or welding, it is recommended that the uncoated areas are repaired. There are a number of repair methods that are available and the galvanizing industry has used and evaluated a wide range of products for over 35 years.
There are a number of factors that have to be considered when repairing hot dip galvanized coatings. These are:
• the size of the area to be repaired
• the ease of use of the repair material
• the performance of the repair method with respect to the required performance of the hot dip galvanized coating.
ZINC RICH PAINT REPAIR
Jotun Protective Coatings products are recommended by the Industrial Galvanizers Australian Galvanizing Division.
For standard duty applications, Jotun Galvanite, a one-pack, air-drying zinc-rich primer can be used. The single pack system has good adhesion to prepared surfaces and can be handled after 2 hours (at 20oC). Coating thickness per coat is typically 45 microns and two coats are required to ensure compatible coating thickness to the hot dip galvanized coatings.
Jotun’s Barrier two-pack epoxy zinc rich system is recommended for heavy-duty applications and full coating reinstatement. The two-pack system is touch dry in 15 minutes and hard dry in less than 2 hours at 20oC. Two coats at 45 microns each are recommended to provide equivalent coatings thickness to hot dip galvanizing.
The recommended procedure for touch-up is as follows:
1. Power tool clean to Australian Standard 1627:2 Class 3 to remove all welding scale, slag and corrosion products.
2. Degrease and remove all surface oil, grease or soil. (This step can be omitted on new welds where no organic contamination of the exposed steel has occurred.)
3. Apply two coats of Galvanite or Barrier to a minimum dry film thickness of 85 microns.
4. Observe good painting practice with respect to weather condition and application conditions. Ensure that the steel surface is above Dew Point prior to application. If below the Dew Point, warm area to ensure surface dryness
5. Adhere to coating manufacturers product data sheets for safety, mixing, pot life, application, overcoating and curing information.
6. If a close colour match is essential, apply a light coat of aluminium paint over repair area after drying. Rub over aluminium paint with a soft rag before drying to blend the repair into the surrounding galvanized coating appearance. NOTE: The aluminium paint is NOT an anti- corrosive coating and does not contribute to the performance of the repair.
METALLIC REPAIR COATINGS
There are various types of metallic repair materials available in the form of alloy `sticks’ which are applied by heating the area to be repaired and melting the repair material onto the area. Early repair sticks had a high lead content and did not reflect the characteristics of a galvanized coating.
Cominco of Canada has developed its Galvaguard repair alloy that has a high zinc content and is easier to use than previous repair alloys.
These repair alloys are best used for repairing welds and similar small areas on horizontal surfaces, that can be easily and quickly heated . A higher level of operator skill is required to effect successful repairs using Galvaguard or similar products. Some of these products contain high levels of tin or lead. They are difficult to apply in other than down-hand positions.
Zinc metal spray is an effective repair method for larger areas, as it will apply a zinc coating of any required thickness. However, successful application requires a high quality surface preparation (Class 3) to ensure good adhesion. Specialised equipment and skilled operators are required to apply zinc metal spray coatings, but they provide a high quality repair solution where larger areas of the item have not been galvanized due to design constraints.
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