Shop Primed Materials: When is too much - too little???
Increasingly, challenges are developing where temporary coatings have been applied off-site to products. These products are scheduled to receive paint systems on-site, but had protective coatings applied off-site by the fabricator to protect the product. Some of these products include clear-coated glue-laminated beams, chromate-passivated galvanized decking, and shop-primed interior and exterior structural steel.Temporary protective coatings often do the job foreseen by the off-site fabricator, but can cause immense problems to the on-site coating applicator. These problems reflect upon the overall quality of the specification, as this information is seldom part of the bidding information given to the coatings applicator. Even if the painting specification is of high quality, this action by the steel fabricator may compromise the expected level of performance foreseen by the painting specifier. This often results in disputes where there will be losers, and may well go to litigation. This is in nobody's best interest with the exception of the lawyers involved!
One example is interior and exterior structural steel. Fabricators often take surface preparation information from their portion of the specification and not from the painting portion of the specification. They then apply a temporary protective coating to protect the steel from rust. The steel arrives at the site with a surface preparation which may not have been specified by the design authority for the coating section. It also may arrive with a primer as the temporary protective coating!
The dilemma facing the coating contractor and the design authority is that if the coating contractor proceeds with the specified system, it will be on top of the temporary protective coating that was applied. As with a chain, the 'weakest link' of a system most often results in coating failure. Even if the steel received the specified preparation (generally SSPC - 6); the temporary protective coating should still be removed in order for the specified system to be applied to the properly-prepared substrate.
An example of a temporary protective coating is the standard for "A Quick-drying One-coat Paint For Use On Structural Steel" that was published in the 1970's as 1-73a by the Institute of Steel Construction, but is still in common use today. Close examination of this "Performance Standard" reveals that it is "for use as a single coat on structural steel.” It "is intended as a temporary coating of structural steel, or for interior exposures such as dry buildings subjected to infrequent condensation." It further says that it "is not recommended as a primer in a multicoat system." The surface preparation requirements call for SSPC SP1 - Solvent Cleaning, and application to a dry film thickness of 1.5 to 2.0 mils. The problem is not so much with the standard, but with its application by the fabricator. The architectural painting specification clearly calls for a different level of surface preparation than called for by the standard, and for an appropriate primer within the specified paint system. When the steel arrives with the level of preparation and the coating called for by the standard used by the fabricator, what is the coating applicator to do? His obligation is to the painting specification, which may well call for a higher level of surface preparation and for an appropriate primer designed for the system that is compatible with the system top coat(s).
Similar conditions sometimes exist with exterior structural steel and steel intended for use in aggressive environments, or environments where moisture or condensation can be a problem.
The MPI is of the opinion that the information in this "Caution Note" should be known by all parties involved (the design authority, the specifier, the steel fabricator, and the coating contractor) in order to minimize any challenges that may arise when coating materials prepared off-site.
Further information (such as compatibility of coatings and 're-coat windows') is available in the System Evaluation sections (i.e. of interior and exterior substrates) in both the MPI Architectural Specification Manual and the MPI Maintenance Repainting Specification Manual available in web and print.