The Perils of Prefinished Steel

BC Place

Specifying that new steel be prepped and primed in the controlled and predictable environment of the fabricating shop can offer many advantages. However, there are schools of thought that go a step further, postulating that complete prefinishing — a.k.a “total shop painting” of the complete system – is the preferred practice. But is it?

Consider this case: 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 2-part gloss polyurethane topcoat) appeared acceptable, with only minor dings and damage. The owner had required paint inspection at the Asian fabrication site, so a mountain of inspection paperwork accompanied the shipment.

BC Place

The design required smaller beams to be cut and welded on site into larger beams – and that’s when the trouble began. During welding/cutting operations, the topcoat started chipping and peeling, revealing the shiny, intact epoxy mid-coat below. To the owner’s dismay, sheet after sheet of topcoat could be hand-scraped off with ease akin to peeling an orange, indicating complete delamination failure between the intermediate and the topcoat.

The Importance of Recoat Windows

The paint manufacturer verified that to assure proper adhesion, the recoat window was a maximum of 72 hours between application of the epoxy intermediate and application of 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.

Since the intermediate coat was pristine with no evidence of dirt or contamination, it was determined that the failure occurred because the intermediate’s 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, prior to applying the topcoat, the shop had taken steps to grind and roughen the fully-cured intermediate and thus provide some tooth for the finish — but the shiny epoxy intermediate surface indicated that no roughening work had been done.

A Costly Solution

BC Place

The only solution was to re-prep and topcoat the beams on site, many of which were already erected. The shiny epoxy intermediate had to be power grinded (a mere scuff-sand was insufficient) to completely eliminate the gloss, and a fresh topcoat of gloss polyurethane applied – all of which would take many hours more than the schedule had originally allotted for ‘spot coating repairs.’

Since spray application wasn’t permitted at this urban jobsite, the affected areas were rolled instead of sprayed. Rolling is never recommended for gloss finishes because gloss accentuates the roller stipple pattern – a pattern that was multiplied with each coat in the system, producing a surface texture that was significantly different in appearance from the adjacent sprayed areas, even when viewed from 100+ feet away. This was aesthetically unacceptable, so the only solution was to roll all the failed beams from top to bottom. In the end, some 80% of the steel was repainted. So while the owner’s intent was to save time and money, in the end, they 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 prefinished in the shop, even overseas? If the project is as simple as “unwrapping the bubble wrap” — e.g. 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 inches – 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.

BC Place

Second, the heat from major cutting and welding work will cause the paint system to peel back 2-3 feet from the cut/weld. All these burned areas must then be grinded back to bare metal and then refinished. It’s likely that a different primer (e.g. an organic zinc instead of an inorganic) 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.

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