Foreword
The following information is given to assist in the general repair and renovation of Hot-Dip galvanized components which have suffered damage in storage or transit or which have been subject to structural alterations post-galvanizing, ie. welding, cutting or drilling etc. The methods stated are designed to renew the cathodic protection and/or cosmetic finish of the original Hot-Dip galvanize coatings, lost when such damage occurs.
In the event that the galvanized materials are to be overpainted with topcoats (ie. white/red etc) and not left in the original galvanized finish, the references to GT Alu Zinc may be omitted.
Tables 1 and 2 give respectively (a) the original recommended Hot Dip galvanize thickness and (b) the suggested repair media thickness.
SURFACE PREPARATION FOR METAL COATINGS
For best results care is needed in the preparation of surfaces prior to application of all Metal Coatings. Where parts have been welded, the 'welds' should be dressed with a grinder and then the whole surface abraded with wire brush/Scotchbrite pad where application of the repair media is required. This gives a 'key' for the metal coating.
The surface should then be cleaned to remove grease, oil and surface deposits, (6047GT Contact Cleaner can be used for this purpose). The repair materials can then be applied. Multiple 'thin' coatings are more practicable than one thick coating. Each coating should be allowed to dry thoroughly between applications. Two to three thin coatings will meet the latest BS/EN/ISO standards for most applications, but can be checked against Table 2 for conformity.
OVERPAINTING OF GALVANIZED PARTS (TOP COATS)
There are many instances where the specification requires the overpainting of galvanized parts. This means that invariably the component will have been hot dip galvanized and where appropriate repaired as described.
Where overpainting of the galvanizing and the repaired areas is required there are features of the paint of which the applicator should be aware. Many paints used Xylene, Toluene, MEK and similar solvents that are listed in the COSHH 1988 regulations as hazardous materials. They are also listed under EPA guidelines as hazardous pollutants and classified toxic.
Conventional high quality brushable paints normally contain 30/40% solids and do not exceed 40/60% solvents. These are quite acceptable as 'primers' providing that individual brushed coatings do not exceed a thickness of 80um. It is essential that each coating be allowed to dry thoroughly before additional coatings are applied. This is to allow complete dispersion of the paint solvents to atmosphere. If this procedure is adhered to, there will be no adverse effect in the repaired areas.
| Material Thickness | Recommended Zinc Film Thickness |
| to 1 mm | 50um |
| 1 to 3 mm | 55um |
| 3 to 6 mm | 70um |
| from 6 mm | 85um |
TABLE 2: Suggested Repair Media Thickness
| Recommended Zinc Film Thickness | Recommended Liquid Zinc Film Thickness |
| 50um | 80+ um |
| 55um | 85+ um |
| 70um | 100+ um |
| 85um | 115+ um |
- Surface Staining & Discoloration
Wire brush the affected area. For cosmetic purposes one or more coats of GT Alu Zinc* may be applied. - Removal of excessive Zinc coating
Spikes and other blemishes can be removed by careful filing. After the removal of surplus Zinc, the coating thickness should be measured in the affected area and compared with Table 1 (recommended galvanizing film thickness). If required, GT Liquid Zinc/spray* may be applied to reinstate the cathodic protection. The coating thickness in the repaired area should conform to Table 2. For cosmetic purposes a coat of GT Alu Zinc* may be applied. - Renovating of Hot Dip Galvanized coatings damaged by wet storage (white rust), cutting, welding or general transit damage (scratches, chips etc.)
White rust deposits should be removed with either a wire brush or 'Scotchbrite' pad. Loose deposits around welded or otherwise damaged areas should be similarly removed. This action also assists in providing a 'key' for the repair media. As previously mentioned, it is important that areas to be treated are also free from oil, grease and dirt.
After removal of white rust, the coating thickness should be measured and compared with Table 1. In all cases, GT Liquid Zinc/spray* may be applied to the damaged areas. The repaired coating thickness should conform to Table 2 (ie. at least 30 microns thicker than the original coating thickness). For cosmetic purposes a coat of GT Alu Zinc* may be applied.
*With the improved corrosion protection of the Zinc-flake formulation of Galv Tech Alu Zinc, 1-2 extra coats of Alu Zinc may be used in place of Liquid Zinc/Spray if preferred.
The responsibility to ensure safe working conditions within the workplace remains with the user. The health hazards and safety precautions required for individual products should be assessed prior to use.
