Specifying the Gold Standard for Color and Protection
The "gold standard" three-part coating system consists of a blast-cleaned surface, a zinc-rich primer, an epoxy build coat and a polyurethane top coat. But many variations are possible, indicating the sorts of decisions that a coating specifier must make.
Where color is required along with corrosion protection, the most widely specified paint coating system consists of a blast-cleaned surface, a zinc-rich primer and an epoxy intermediate, along with a top coat of polyurethane. This is sometimes referred to as the "gold standard" system.
Polyurethane provides excellent gloss, color and durability. Intermediate mid-coats are based on epoxies, which have excellent durability, adhesion and toughness; but over time, they chalk and normally require top coating. Primers can be zinc-rich, either epoxy zinc or inorganic zinc, or zinc-free epoxy.
There are a number of possible systems, depending on:
- The environment
- The required durability
- Whether it is maintenance or new work
- Whether there are restrictions on blasting or spraying
For most new work, a system consisting of a near-white blast (SP-10, NACE 2 or Sa 2.5), 75 microns (3 mils) of zinc-rich primer, 125 microns (5 mils) of epoxy mid-coat, and 50 microns (2 mils) polyurethane top coat is a widely-specified system that's ideal for most environments. But many variations to this system are possible, indicating the sort of decisions a specifier must make. Let us look at the various stages and systems.
New Work: Surface Preparation and Priming
For most new work, the surface would be blast cleaned to a near-white blast. For atmospheric work, the minor durability improvement gained by a white blast (SP-5, NACE #1, Sa3) would not be worth the extra cost and delay.
Either inorganic zinc or epoxy zinc can be specified as the primer, but previous research indicates that in a multi-coat system, there is little difference in the performance of epoxy zincs and inorganic zincs.1 Because epoxies are easier to apply and top coat, and do not have the curing limitations of inorganic zincs, they are normally recommended.
If use of zinc is restricted, such as in some refineries or in acidic environments, a zinc-free epoxy primer would be specified, again to 75 microns. Such primers normally contain zinc phosphate inhibitive pigment, barrier pigments, or both. However, zinc-rich primers provide far better protection to edges and damaged areas, and should be specified wherever possible in atmospheric applications. (For further discussion on selecting the best coating alternative read Anti-Corrosion Coatings for Different Service Exposures.)
If blast cleaning cannot be carried out, hand or power-tool cleaning to SSPC SP2, SP3, SP11, or ISO 8501-1 St3 is usually specified.
With a compromised surface, an epoxy mastic (surface-tolerant epoxy) is normally applied to a minimum dry film thickness of 125 microns (5 mils) if applied by spray, and to 75 microns (3 mils) if brush application is required. Primers should not be applied by roller. Epoxy mastic would also be the primer specified for spot repair, even if spot preparation can be carried out by blast cleaning, because it is likely to be compatible with existing weathered coatings.
Mid, Intermediate or Build Coating
A mid-coat is normally applied to build up thickness and ensure good coverage. A three-coat system will always provide better protection than two will, because there is better coverage of critical areas, such as edges and corners.
A build coat of 125 microns (3 mils) of standard epoxy is normally sufficient, although this can be increased to 200 microns (8 mils) for very severe environments. The color should be different from the primer or top coat, and edges should be stripe coated if this was not done at the primer stage.
If spraying is not allowed, high build is not possible, and the mid-coat would be specified to 75 microns (3 mils). Epoxy mastic can be used as the mid-coat for maintenance, brush or roller application. The mid-coat may be dropped in less severe environments, for shorter life, or to reduce costs.
A thin top coat of polyurethane is applied to provide color and gloss retention, because epoxies exposed to atmospheric environments tend to chalk. The glossy finish also minimizes dirt buildup and allows for easier wash-down.
Because polyurethane is expensive and has more restrictive health and safety requirements than epoxies, it is usually limited to the top coat. (For more on safety, see Writing Safety Into Your Coating Specification.) The top coat is normally specified to 50 microns (2 mils), but for some colors such as certain yellows, a second coat is required for opacity.
Where spraying is not possible, high-build polyurethanes should preferably be applied up to 75 microns (3 mils) by brush or roller. High-build polyurethanes to this thickness are also widely specified when spraying is allowed.
The “gold standard” three-coat system has provided quality protection and good appearance on steel structures for nearly 50 years. New coatings have appeared that claim better performance along with safer or cheaper application. But the current system has an inevitable track record and it will continue to be the system to which all others are compared into the future.
Written by Rob Francis | Consultant, Aurecon Materials Technology Group
Ph.D., Corrosion Science, University of Melbourne
Rob Francis has 40 years’ experience in the corrosion and coatings fields, with emphasis on steelwork in atmospheric and marine environments, as well as ferrous metals, technical training and quality assurance.