THURSDAY, SEPTEMBER 12, 2013
Editor's Note: This article was written by MPI (Master Painters Institute) and is reprinted with permission.
The majority of new construction projects specify that structural steel components be prepared and primed by the steel fabricator prior to delivery to the job site.
However, time and time again, we see situations where the steel they send to the jobsite is nowhere close to being up to spec—and the problem is exacerbated when that fabricator is not local and may be hundreds of miles (or oceans) away.
Projects where the steel is to receive finish coats applied at the jobsite are the greatest concern.
Bad Prep and/or Bad Primer
The problem with the newly delivered steel can be in the quality of the surface preparation.
The architect may have specified SSPC-SP 6 Commercial Blast Cleaning, but the steel arrives with minimal surface preparation—power tool cleaning at best. Or the issue may be the primer, where, instead of the specified anti-corrosive primer (such as products approved under MPI #79 Primer, Alkyd, Anti-Corrosive or MPI #76 Primer, Alkyd, Quick Dry), the fabricator substitutes an inexpensive fast-dry shop primer.
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| Photos: MPI |
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Corrosion can begin almost immediately when the wrong primer is used or when specifications conflict or are not met. Clarification and inspections early in the process can help head off problems. |
These fast-dry shop primers are intended only for temporary protection during shipping and for steel that is going to be buried in concrete and/or never seen. With a DFT in the range of ½ mil, they cannot withstand exposure for extended periods of time, nor are they designed to be topcoated.
Often, both the prep and the primer are off-spec.
A Series of Unfortunate Surprises
Unfortunately, the problem is likely to be noticed long after the steel is delivered to the jobsite and the trusses and roof are installed. The general contractor is happy to see the roof on and everything on schedule, so he calls the painting contractor in to apply the finish coats.
If a paint inspector has been hired, he comes in to find corrosion and other surface defects on the new steel and, upon examination, discovers the poor quality surface preparation and/or incorrect primer.
If inspection hasn't been specified, the painting contractor finds this unpleasant surprise waiting for him when he begins the work.
The contractor knows that painting the steel as is will almost certainly lead to premature failure (along with the likelihood of blame falling on his shoulders), so instead of proceeding with the job, he brings the compromised surface condition to the attention of the GC, who ultimately brings the problem to the owner.
Difficult Decisions
Now, the owner has a difficult decision to make. It is highly likely that the steel paint system will fail prematurely, resulting in a costly series of repainting efforts that may never fix the problem.
But what are his options? Once the components are on site, it can be difficult for the general contractor to send them back or have the time to redo the steel.
The steel will either have to be disassembled and shipped to a local fabricator for blasting and application of the correct primer, which can be a costly and time-consuming process. Or the steel will need to be re-prepared and repainted on site.
Since abrasive blasting on site is generally not feasible, extensive power tool cleaning may be required to remove the primer and create a suitably clean surface. Also, since LEED or local mandatory environmental requirements are likely to forbid the application of MPI #79 or other solvent-based primers on site, a waterborne primer such as products approved under MPI #107 may be required. And these may need two coats to attain the required DFT, adding more cost and time delays.
No matter which option the owner takes, this is a very costly situation where even the most conscientious owners have been known to relax their requirements and allow the job to continue with the not-to-spec steel, simply because of time and cost constraints.
Why does this happen again and again? And how can these problems be avoided?
The Solution is Prevention
Sometimes, the problem stems from conflicts within the specifications.
Surface preparation and coating requirements may be found in two different places in the project specs. Section 05500 covers the steel fabrication and includes requirements for surface preparation and priming that are consistent with the environment the steel will be subjected to; Section 09900 covers the finish painting requirements.
It is critical that there be continuity between the two.
If steel is to receive a finish coating system in 09900, then the surface preparation and priming described in 05500 must be suitable—commonly, an SSPC-SP 6 Commercial Blast Cleaning and a corrosion-resistant primer that's part of a two- or three-coat system, such as MPI #79 Primer, Alkyd, Anti-Corrosive or MPI #76 Primer, Alkyd, Quick Dry. Or, for high-performance systems, a zinc-rich or epoxy primer such as MPI #20 or MPI #101.
Conflicting Requirements
The conflict arises if Section 05500 cites standards designed for steel that is meant to be neither topcoated nor seen—steel that will either be encased in concrete or used as ceiling trusses that will be hidden from view.
These standards require minimal surface preparation—SSPC-SP 3 Power Tool Cleaning at best—and permit use of primers designed only to protect the steel from flash rusting during transportation, not protection from long-term exterior exposure on a jobsite or as part of a coating system.
Our thinking is: When the fabricator and general contractor review the specification and related sections, care must be taken to assure that the surface prep and priming requirements described in Section 05500 are consistent with the finishing requirements in Section 09900. A few questions to the paint inspector and architect could resolve any discrepancies before they turn into problems.
When Specs aren't Met
More often, the poorly primed steel is a result of the fabricator's simply not having met the specs. The fabricator that was awarded the contract has no capability for proper abrasive blasting, or doesn't stock the required primer, or both.
Even if inspection is called for, if the inspector is asked only to review the steel at the fabricating shop after it’s done and ready for shipping, it’s already too late.
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Why inspection is important: Here are steel components prepared by two different fabricators to the same exact spec. One job had inspection; the other did not. Not quite the same, are they? |
Alternatively, we recommend that the paint inspector on the job review the specifications for the steel before the project starts.
If the steel is being produced out of town, the inspector can call the producer to thoroughly review the requirements. The inspector should alert the producer that the inspector will be checking the condition of the steel when it arrives on site, and that the shipment will be rejected and not come off the truck if it is not up to spec.
But the best way to assure that steel arrives at the jobsite properly blasted and coated is for the owner or architect to ask that the paint inspector be present at the fabricating shop during prep and priming.
The inspector can document and verify that the work is acceptable and be empowered to stop the project if the work is proceeding off-spec.
The owner will pay a bit more for the extra inspection time and travel costs, but this investment is a drop in the bucket compared to the nightmare no-win scenario described above.
Tagged categories: Commercial Construction; Corrosion protection; Good Technical Practice; Master Painters Institute (MPI); MPI Approved Product; Primers; Quality control; Shop-applied coatings; SSPC; Steel; Surface preparation
