The corrosion potential for damaged areas on hot dip galvanizing is much less than it is with paint systems. The zinc adjacent to the damaged area will provide cathodic protection to the damaged area. The cathodic protection will also prevent underfilm corrosion of the adjacent coating which will prevent the area of concern from becoming larger. If the damage is the result of severe handling due to transportation or erection, then typically a zinc iron alloy layer will remain which will further protect the area.

The best repair procedure is metallizing to ASTM A780 A3. However, it requires expensive equipment, trained operators and strict adherence to proper procedures such as preheating. It is rarely required because of the low corrosion potential of damaged areas.

We recommend the following specification:

Areas To Be Repaired

1. Any area with a remaining alloy layer larger than 30mm in diameter or more than 10mm in width if longer than 30mm.
2. Any area where the base metal is exposed to a width of more than 4mm.

Repair Using Zinc ‑Rich Paints According To ASTM A780 A2

1. If surfaces to be reconditioned still have the zinc alloy layer present, the surface shall be clean, dry, and free of oil and grease.
2. If damaged area has all zinc removed, blast clean the surface in accordance with SSPC‑SP5, white metal. Where circumstances do not allow blast cleaning, it is permissible to power disk sand areas to be repaired to bright metal. To ensure that a smooth reconditioned coating can be effected, surface preparation shall extend into the undamaged galvanized coating.
3. Spray or brush apply the zinc‑rich paint to the prepared area. Apply the paint as recommended by the paint manufacturer employing multiple passes to achieve a minimum total dry film thickness of 100 microns. Paint must overlap slightly onto the area around the patch but do not over paint so much as to exaggerate the size of the repaired area.
4. Take measurements of the coating with a magnetic gage to ensure that the applied thickness is as specified.

Zinc-rich paint must be 94% metallic zinc dust by weight. Some paints that meet this requirement are Devcon Z (Century Sales) and Galvicon (Soldan Fence, Edmonton).