THURSDAY, SEPTEMBER 26, 2013
Editor's Note: This article was written by PQA Inspector Dave Lick and is reprinted with permission from the MPI (Master Painters Institute) newsletter. MPI content describes best practices for commercial, institutional, and light industrial painting.
Specifying that new steel be prepped and primed in the controlled and predictable environment of the fabricating shop can offer many advantages. Some schools of thought go even a step further, postulating that complete pre-finishing—also known as "total shop painting” of the complete system—is the preferred practice.
But is it? Consider this case.
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| Photos: MPI |
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The finish gleamed throughout two inspections. But when the cutting began, so did the trouble. |
The design for a large new public arena called for exceedingly large exposed beams and structural members.
The owner believed he could save time and money by having the steel fabricated and pre-finished in Asia. The overseas fabricator’s bid was lower than bids from domestic vendors, and having the steel arrive pre-finished would expedite the construction process.
The North American paint inspector met the steel coming off the ship at the local port. The specified coating system (inorganic zinc primer, high-build epoxy intermediate, and two-part gloss polyurethane topcoat) appeared acceptable, with only minor dings and damage.
The owner had also required paint inspection at the Asian fabrication site, so a mountain of inspection paperwork accompanied the shipment.
The design required smaller beams to be cut and welded on site into larger beams—and that’s when the trouble began. During welding and cutting operations, the topcoat started chipping and peeling, revealing the shiny, intact epoxy mid-coat below.
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The peeling topcoat is removed with hand scrapers. Sheet after sheet came off, like peeling an orange. |
To the owner’s dismay, sheet after sheet of topcoat could be hand-scraped off with the ease of peeling an orange, indicating complete delamination failure between the intermediate and the topcoat.
The Importance of Recoat Windows
The paint manufacturer had verified that to assure proper adhesion, the recoat window was a maximum of 72 hours between application of the epoxy intermediate and the polyurethane topcoat.
The inspection documents from the shop were then consulted. But much to the North American inspector’s surprise, there was no record of the actual recoat window time.
Because the intermediate coat was pristine with no evidence of dirt or contamination, it was determined that the failure had occurred because the recoat window had been exceeded. By the time the polyurethane topcoat was applied, the intermediate epoxy had cured beyond the topcoat’s ability to bond to it.
The failure still could have been avoided if the shop had taken steps before topcoating to grind and roughen the fully cured intermediate and thus provide some tooth for the finish. But the shiny intermediate surface indicated that no roughening work had been done.
A Costly Solution
The only solution was to re-prep and topcoat the beams on site, many of which were already erected.
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This coating repair was done in the air, as many beams had already been erected. |
The shiny intermediate had to be power-grinded (abrasive blasting was out of the question on this job site, and a mere scuff-sand was insufficient) to completely eliminate the gloss, and a fresh topcoat of gloss polyurethane applied. All of this would take many hours more than the schedule had allotted for "spot coating repairs."
Furthermore, because spray application wasn’t permitted at this urban jobsite, the affected areas were rolled. Rolling is never recommended for gloss finishes, because gloss accentuates the roller stipple pattern.
That pattern was multiplied with each coat in the system, ultimately producing a surface texture that was significantly different in appearance from the adjacent sprayed areas, even when viewed from more than 100 feet away. This was aesthetically unacceptable.
The only solution was to roll all of the failed beams from top to bottom. In the end, some 80 percent of the steel was repainted.
So while the owner’s intention was to save time and money, in the end, he got the opposite: well over $1 million in added expense to replace the failed topcoat and havoc wreaked on the construction schedule.
Lessons Learned
So, is it ever a good idea to specify steel pre-finished in the shop, even overseas?
If the project is as simple as “unwrapping the bubble wrap”—if, for example, the item can be easily handled and doesn’t require a lot of modification, such as handrails, some staircases, and gutters—it can certainly be advantageous to have parts completely finished off site.
But when you’re dealing with steel that’s 1.5 to 2 inches thick, and the design requires major cutting, welding and modifications on site, the benefits start to diminish.
First, if there’s a problem with the shop coating work, as there was on this project, you can’t send the steel back. Once it’s on the boat, the construction team has no recourse but to move ahead when it arrives.
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Welded areas are ground back to prep for the new primer. In the end, repairing and recoating the beams cost the owner an additional $1 million. |
Second, heat from major cutting and welding will cause the paint system to peel back two to three feet from the cut or weld. All these burned areas must be grinded back to bare metal and then refinished.
It’s likely that a different primer (an organic zinc instead of an inorganic, for example) that’s more tolerant to field application and imperfect surface preparation must be substituted. And that new primer must be compatible with the rest of the system, so more changes to the spec might be required.
So in the end, when it comes to pre-finishing steel in the shop, sometimes the owner learns the hard way that by the time you ship the steel, truck it to the jobsite, assemble, cut, weld, and modify, the amount of re-work and touch-up required nullifies any advantages gained.
Tagged categories: Adhesion; Coating failure; Delamination; Epoxy; Good Technical Practice; Grinders; Master Painters Institute (MPI); Shop-applied coatings; Structural steel; Surface preparation; Topcoats; Welding
