Paint Systems For Galvanizing

SELECTING THE RIGHT PAINT

There are three main steps required to select the right paint to apply over any fabricated, hot dip galvanized structural steel article, once it has been determined the design lends itself to painting in a manner which can achieve uniform paint application. They are:

  1. Identify the environment (Corrosivity Environment – A Quick Guide)
  2. Determine the service life required for the painted structure (see below)
  3. Select the appropriate painting system from the Service Requirement Guide (see below)

If there is any doubt, you should always choose the more aggressive corrosivity environment. The Durability of Galvanizing Estimator can be used to compare the durability of different duplex systems (paint over galvanizing).

Choosing the service life is often a matter of cost. If the paint system does not last long enough, rectification could be expensive. In the same manner; as the effectiveness of paint systems increase, the materials and application costs rise. AS 2312.1 Appendix E provides guidelines on the economics of corrosion protection and the specifier is urged to consider not only the initial cost but also the lifetime cost of any chosen corrosion protection system.

PAINTING SYSTEMS

The following provides guidance on paint systems suitable for use in industrial and commercial situations under six different “service requirements” as follows:

Service Requirement 1 Low corrosivity conditions & medium term service.
Service Requirement 2 Low corrosivity conditions & high resistance to wear & trafficking/long term service.
Service Requirement 3 Medium corrosivity conditions & high resistance to wear & trafficking/long term service.
Service Requirement 4 High corrosivity conditions & high resistance to wear & trafficking/long term service.
Service Requirement 5 Very high corrosivity conditions & high resistance to wear & trafficking/long term service.
Service Requirement 6 Specific industrial chemical or solvent exposure.

 

Notes

 

  1. Service life: The paint systems listed in this guide provide for either medium or long term service lives, which are typically 5 to 10 years or 10 to 15 years respectively, for a maintenance repainting cycle to retain aesthetic performance.

    NOTE: In this Guide we have assumed aesthetic performance is critical to the design. AS/NZS 2312.2 Section 7.5.2 and Table 7.2 provide information on expected durability of the duplex coated article if aesthetic performance is not critical and professional advice should be sought for preparation of an appropriate paint system specification.

  2. The corrosivity condition assessment is based upon AS 4312, which provides excellent guidance for the determination of the general Australian macro environment with respect to corrosivity zones and the guidelines of AS/NZS 2312.2, which essentially defines corrosivity in terms of distance from the seacoast. (Corrosivity Environment – A Quick Guide) provides assistance on choosing the corrosivity environment.
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    NOTE: AS/NZS 2312.2 – 2014 includes the new extreme (CX) corrosivity zone. The GAA has no paint system recommendation for category CX at the time of preparation of this guide.
  3. Application should be strictly in accordance with the paint manufacturer’s written instructions and the relevant recommendations of AS/NZS 2311 in the case of Service Requirement 1 and to AS/NZS 2312.2 in the case of Service Requirements 2 to 6.
  4. For all but Very Low and Low Corrosivity Service Requirements it is recommended that a brush applied stripe coat be applied to corners, welds and other sharp edges to ensure full thickness coverage. Indeed, for High and Very High Corrosivity Conditions it is critical. This is because, unlike hot dip galvanizing, paint tends to shrink away from sharp edges.

 

Service requirement 1

C1 (very low) & C2 (low) corrosivity conditions – Medium term service – Decorative finish
Clean &
Degrease
1 coat of Latex Primer to AS3730.15 or
APAS0134
1 coat of 100% Acrylic Gloss Latex to AS3730.10 or APAS0280/1 1 coat of 100% Acrylic Gloss Latex to AS3730.10 or APAS0280/1
Notes:
1 AS/NZS 2312.2 Corrosion Categories C1 and C2. It also includes Category C3, provided the structure is totally exposed to the cleansing influence of rainwater or is subject to an appropriate regular hosing with fresh water, as unwashed areas in Category C3 can be quite corrosive.
2 Typically 5 to 10 years maintenance repainting cycle to retain aesthetic performance.
3 For enhanced resistance to wear and trafficking and staining adopt Service Requirement 3.
4 Flat, Low Gloss and Semi-Gloss 100% acrylic latex finishes may be also options, as aesthetics demand.
5 Application should be strictly in accordance with the paint manufacturer’s written instructions and consistent with the relevant recommendations of AS/NZS 2311 Guide to the painting of buildings.

 

Service requirement 2

C1 very low & C2 low corrosivity conditions – Long term service – Decorative finish
Minimum system DFT 175 µm
Clean 1 coat of 2 pack inhibitive epoxy primer (minimum DFT 75 µm) 2 coats of 2 pack polyurethane or 2 pack acrylic (minimum DFT 50 µm/coat)
Degrease
Notes:
1 AS/NZS 2312.2 Corrosion Categories C1 and C2. It also includes Category C3, provided the structure is totally exposed to the cleansing influence of rainwater or is subject to an appropriate, regular hosing with fresh water, as unwashed areas in Category C3 can be quite corrosive.
2 Typically 10 to 15 years maintenance repainting cycle to retain aesthetic performance.
3 For enhanced resistance to wear and trafficking or staining adopt Service Requirement 3.
4 Powder Coatings covered under Service Requirement 3 are also appropriate.

 

Service requirement 3

C3 medium corrosivity conditions – High resistance to wear & trafficking/long term service
Decorative paint finish
Minimum system DFT 175 µm
Clean & Degrease Brush Blast 1 coat of 2 pack inhibitive epoxy primer (minimum DFT 75 µm) 2 coats of 2 pack polyurethane or
2 coats of 2pack acrylic
(minimum DFT 50mm/coat )
Powder coating
Clean, coat and cure in accordance with the powder coating manufacturers written instructions using the appropriate polyester powder coating to a cured thickness of not less than 70 µm.<
Industrial finish
Minimum system DFT 225 µm
Clean & Degrease Brush Blast 1 coat of 2 pack inhibitive epoxy primer (minimum DFT 75 µm) 1 coat of high build 2 pack epoxy (minimum DFT 150 mm)
Notes:
1 AS/NZS 2312.2 Corrosion Category C3, except in tropical locations where Service Requirement 4 is more appropriate. It also includes Category C4 locations, provided the structure is totally exposed to the cleansing influence of rainwater or is subject to an appropriate, regular hosing with fresh water, as unwashed areas in Category C4 can be very highly corrosive.
2 Wear & trafficking is defined as surfaces subject to mild abrasion and scuffing, such as occurring in public thoroughfares, kitchens and other work areas.
3 Typically 10 to 15 years maintenance repainting cycle to retain aesthetic performance.
4 For enhanced performance or in the more corrosive areas within the Category C3 a higher minimum total cured film thickness may be required.

 

Service requirement 4

C4 high corrosivity conditions1 – High resistance to wear & trafficking/long term service
Decorative paint finish
Minimum System DFT 350 µm
Clean & Degrease Brush Blast 2 coats of high build of 2 pack epoxy (minimum DFT 250 µm) 2 coats of 2 pack polyurethane or 2 pack acrylic (minimum DFT 50 µm/coat)
Industrial paint finish
Minimum System DFT 350 µm
Clean & Degrease Brush Blast 3 coats 2 pack high build MIO epoxy
Notes:
1 AS/NZS 2312.2 Corrosion Category C4. These include tropical coastal service and indoor swimming pools.
2 Wear & Trafficking is defined as surfaces subject to mild abrasion and scuffing, such as occurring in public thoroughfares, kitchens and other work areas.
3 Typically 10 to 15 years maintenance repainting cycle to retain aesthetic performance.
4 Where decorative finish is not required polyurethane or acrylic may be replaced by a further coat of high build 2pack epoxy of equivalent thickness.

 

Service requirement 5

C5 very high corrosivity conditions1 – High resistance to wear & trafficking/long term service
Decorative paint finish
Minimum system DFT 400 µm
Clean & Degrease Brush Blast 1 coat of inhibitive 2pack epoxy primer (minimum DFT 75µm) 1 or more coats of high build 2 pack epoxy (minimum DFT 225 µm) 2 coats of 2 pack polyurethane or acrylic (minimum DFT 50µm/coat)
Industrial finish
Minimum system DFT 400 µm
Clean & Degrease Brush Blast 1 coat of inhibitive 2 pack epoxy primer (minimum DFT 75 µm) 3 or more coats 2 pack high build MIO epoxy
Notes:
1 AS/NZS 2312.2 Corrosion Categories C5. It also applies in a few aggressive industrial areas.
2 Wear & Trafficking is defined as surfaces subject to mild abrasion and scuffing, such as occurring in public thoroughfares, kitchens and other work areas.
3 Typically 10 to 15 years maintenance repainting cycle to retain aesthetic performance.
4 Polyurethane finish preferred unless OH&S considerations prohibit isocyanates.
5 Alternatives to galvanizing should be considered, particularly in wear and tear conditions.

 

Service requirement 6

Specific industrial chemical or solvent exposure
Typical minimum system DFT 400 µm
Clean & Degrease Brush Blast 1 coat of inhibitive 2 pack epoxy primer (minimum DFT 75 µm) Finish coat subject to manufacturer?s technical advice
Notes:
1 While the requirements for surface cleaning and brush blasting are mandatory, the specific paint system and its total system thickness are dependent upon the type and concentration of the chemical/solvents exposure and the manufacturer?s advice.
2 Typically 10 to 15 years maintenance repainting cycle to retain aesthetic performance.
3 Typically polyurethanes are specified for resistance to acids and organic solvents and epoxies for resistance to alkalis. Where strong acid or alkali contact is envisaged, alternative construction materials should be considered.
4 Alternatives to galvanizing should be considered, particularly in acid or alkali conditions.

 

TYPICAL PAINTS

Major  paint suppliers have provided the GAA with specific product recommendations to meet the Service Requirements in this guide.  The supplement Duplex Coatings – Paint Product Guide (under development) lists the supplier’s products with notes and provides their contact information if further support is required.