Shop Versus Field Painting: An Exemplum

Introduction

According to Wikipedia, an exemplum (Latin for “example”) is a moral anecdote, brief or extended, real or fictitious, used to illustrate a point. While I wouldn’t consider deciding whether to perform painting work in a shop or in the field a “moral issue,” this story-telling approach is intended to provide a more interesting way to learn about the variety of considerations for three different approaches an owner can consider when needing to protect new steel from corrosion using protective coating systems. I hope you enjoy the story, but more importantly that it helps you weigh your options more knowledgably when faced with similar circumstances.

Facility Owner Options for Preparing & Coating New Steel

Before beginning the story, it is important to understand an owner’s options for preparing and coating new steel. The three options described below are based on a traditional 3-coat system; with minor modifications the same approaches can be applied to 2-coat and 4-coat systems that are less common but can be chosen based on the prevailing service environment and the generic coating system selected. While a fourth option is total field painting, it is very unusual to fabricate new steel and transport it to the field to be abrasive blast cleaned and coated.

Option A – Shop/Field/Field: Apply the primer in the shop, transport the steel to the project site, erect the steel, then apply the mid-coat and topcoat in the field.

Option B – Shop/Shop/Field: Apply the primer & mid-coat in the shop, transport the steel to the project site, erect the steel, then apply the topcoat in the field.

Option C – Total Shop Painting: Apply all coats in the shop, transport the steel to the project site, erect the steel, then touch-up erection damage and connections.

Shop Types and Certification Considerations

The new steel may be fabricated and coated in the steel fabrication shop, or it may be fabricated then transported to a “blast & paint” shop, that prepares and coats the fabricated steel and transports it to the project site. There are also different types of shops, including enclosed shops, covered shops and open shops; the latter two offer much less control over prevailing ambient conditions and airborne dust.

Since preparing and coating steel with high performance coating systems is not a simple task and can produce serious consequences if done incorrectly, the owner may consider specifying the type of shop (from the list above) and requiring certification as a prerequisite to submitting a bid. If certification is invoked by the owner, the American Institute of Steel Construction (AISC) Sophisticated Painting Endorsement (SPE) primarily applies to steel fabrication shops that paint, and the SSPC: Society for Protective Coatings Qualification Procedure 3 (SSPC-QP3) primarily applies to shops that blast and paint, but do not fabricate. There is reciprocity between the two certification programs and they have developed a joint certification standard, AISC-420-10/SSPC-QP 3.  The joint standard also describes the three types of shops mentioned above.

Setting the Stage

With that as background, let’s begin our story. A state turnpike toll agency (Owner) located in the northeastern US is replacing a very old overpass bridge structure carrying a two-lane state route over six turnpike traffic lanes (3 east-bound 3 west-bound). The state route and turnpike are both heavily traveled, and the overpass structure is located near a prestigious neighborhood with a country club and PGA-quality golf course nearby. Additionally, the turnpike is modernizing a service plaza approximately 1 mile east of where the new bridge will be erected. The turnpike anticipates letting a single contract for the fabrication and painting of the steel for both the bridge and service plaza. Since the Turnpike agency is located in a NEPCOAT (Northeast Protective Coatings Committee) state, they anticipate choosing a coating system from List A (inorganic zinc-rich primer/polyamide epoxy mid-coat/acrylic polyurethane finish coat) for all fabricated steel, for both the bridge and service plaza. This system will require SSPC-SP 10, Near-White Abrasive Blast Cleaning and a 2-3 mil surface profile. The finish coat color selected by the local community is a deep forest green with a high-gloss sheen.

The Toll Agency hired an architect-engineering (AE) firm to help them prepare the specifications for the two projects. The AE firm, represented by James is comfortable with a traditional approach of fabricating, preparing and priming the steel in the shop, then applying the mid-coat and finish coat post-erection. The Owner (Steve) wants to have all coats applied in the shop to eliminate the need for any field painting beyond touch-up. The Owner requested a face-to-face meeting with the AE firm to explore the pros and cons of both approaches so that the contract documents can be prepared and sent to pre-qualified bidders. Let’s listen in on their meeting to learn about the perspectives of each party.

Meeting of the Owner and AE Firm

Steve: “Thanks for agreeing to meet with me on this James. We really need to be on the same page before moving forward with designing this project. Let’s discuss the bridge steel first, then we can discuss the steel for the service plaza remodel since circumstances are a bit different. You said that you were questioning our preference to have all coats applied in the shop.  There are several reasons for wanting all coats applied in the shop:

First, the conditions of temperature and humidity are much better controlled in an enclosed shop and the schedule will not be adversely impacted by rain events, which are frequent during the spring and summer months when the bridge steel will be erected. Also, we frequently experience high humidity at the bridge site during those months and the epoxy mid-coat is sensitive to moisture during curing.  We had a problem in the past when high humidity caused an amine exudate to form on the mid-coat leading to peeling of the finish coat. While we recognize that the exudate can be removed by washing prior to applying the topcoat, it’s another step in the process and a risk that we don’t want to take.

Second, access to all surfaces for preparation, application of all coats and inspection is much better in the shop than in the field, and crews are accustomed to protecting the coated surfaces during loading, transportation and erection of the coated steel. Don’t get me wrong, I fully anticipate that there will be touch-up required in the field, post-erection, to repair damaged areas and coat connections after bolting, but those touch-up areas will blend-in nicely over time. I have no doubt that that localized touch-up will be a lot less disruptive than applying one or two coats in the field.

And since traffic will continue to pass beneath the structure on one or two lanes during construction, if we have to apply full coats in the field, rigging will be complicated, and we’ll need containment to collect wash water and prevent overspray claims during application and prevent contamination of the wet coating. This will add significant costs to the project.

I also have concerns that if the field contractor doesn’t get the finish coat applied to all surfaces before the painting season ends, we’ll have to mobilize again next year.  If so, and the last coat applied in the shop is epoxy, it will be exposed to sunlight over the winter causing it to chalk, and will be subjected to dirt, debris, and deicing salts that will accumulate on the surface. This will require washing the surfaces in the spring, and maybe even scarifying prior to topcoating; otherwise we could experience intercoat delamination problems down the road. And if the shop coat is just the inorganic zinc, any significant project delays will result in oxidation of the inorganic zinc-rich primer, which could also interfere with mid-coat adhesion if not adequately removed. There are just too many unknowns to risk applying one or two coats in the field post-erection.”

James: “Wow Steve, you’ve done your homework on this. But let me explain why you should consider applying the mid-coat and finish coat in the field, or perhaps just the finish coat after the steel is erected. And I agree, let’s focus on the bridge steel first

The inorganic zinc-rich primer that you want to use is a good choice, but recognize that it will need moisture to cure, and the primed steel could sit in the shop’s paint bay for days while the zinc cures long enough to apply the epoxy intermediate. If many days of curing between coats are required, some shops won’t have the floor space to handle this, and I can almost guarantee this won’t be the only job in the shop. I’m concerned that the paint crews might rush through the paint operation or move the steel to another area before adequate drying time to free-up valuable floor space.”

Steve: “I have no problem switching to NEPCOAT List B and using an organic (epoxy) zinc-rich primer to eliminate your concerns over inadequate moisture in the shop and potential delays with overcoating the primer. I don’t think temperature will be an issue in the shop, and the recoat times are substantially less. And I hear the organic zinc-rich primers are easier to apply and don’t mud crack.”

James: “That’s a consideration, but remember our bolted connections are Class B and we need a primer that has a Class B rating for the faying surfaces. Some, but not all organic zinc primers have the rating we need. And the zinc loading is higher with the inorganic zinc, which will provide greater galvanic protection, at least theoretically. Switching primers is certainly a viable option, but before we go that route, can we compromise and stay with the same system but consider applying the primer and mid-coat in the shop? This will free up shop space more quickly by dropping the time for applying and curing the finish, and reduce some of the impact on shipping by getting the steel out the door quicker. Then just the finish coat would be applied post-erection. Handling damage will be much less of an issue since an entire coat is being applied in the field. And we won’t sacrifice any long-term protection.

I know this sounds odd coming from an engineer, but I am very concerned about aesthetics. You’re correct that areas touched-up with finish coat normally blend in over time, but there are two characteristics of the topcoat you have selected that make touch-up problematic: color and sheen. While lighter shades blend-in well, dark colors do not, and the high-gloss finish will highlight every touch-up area. While corrosion protection won’t be sacrificed, aesthetics will, and you know how particular that community is.

Another thing to keep in mind is the deck pour. I haven’t seen a project yet where the concrete pour hasn’t resulted in cement scum on the painted steel girders. I know they should immediately wash it off, but that isn’t always the case and once it dries it is very difficult to remove. The bottom line is that the finish coat, and perhaps the underlying coats as well, will be damaged when the scum is abraded off the surface. This will result in a lot of touch-up, or the application of another full coat of finish.”

Steve: “You’ve given me a lot to think about. Let’s discuss the service plaza remodel to see if that helps us decide on an approach. I realize that we don’t have to use the same approach for both the bridge steel and the service plaza steel, but it may help clarify things. As you know the architect has designed the remodel to look a lot more modern and there will be a lot of exposed steel in an atrium ceiling consisting of a lot of small angles and intersecting pieces with bolted connections. That ceiling is 50 feet above floor level in some places. In this case I’m convinced that putting all three coats on in the shop is the way to go.  Applying a complete, quality finish coat to these complex configurations after the steel is erected will be too difficult, and it has to look good.  I think it’s best to limit the amount of on-site painting work to just touch-up of damage and connection areas where the cleaning and painting can be focused. And remember we aren’t dealing with a dark green, high-gloss topcoat. The architect has recommended bright white with a satin finish to take advantage of natural light and realize some energy savings.”

James: “All good points, but I’ve seen the blueprints and there are a lot of connections that will require touch-up and the erection of a lot of small pieces that will be prone to edge damage. I am envisioning extensive touch-up that will require a lot of feathering and “artist brush” work.  Since the area will be visible to the public below, I believe a complete, seamless finish should be applied. Maybe we compromise here as well and apply two coats in the shop and the finish coat post-erection.”

Epilogue

Both James and Steve have researched the advantages and limitations well, and both have good arguments to support their respective positions. What would you do? For the bridge steel would you elect Option A (apply the primer in the shop, transport the steel to the site, erect the steel, then apply the mid-coat and topcoat); Option B (apply the primer & mid-coat in the shop, transport the steel to the site, erect the steel, then apply the topcoat); or Option C (apply all coats in the shop, transport the steel to the project, erect the steel, then touch-up erection damage and connections)? And would you elect the same option or take a different approach for the exposed steel in the atrium of the service plaza? There is no wrong approach; the key is to understand and fully consider all the challenges that come with fabricating, preparing and coating new steel before deciding on an approach.

A 24-minute podcast released earlier this year on KTA University features Ken Trimber as moderator, while Doug Reardon advocates total shop painting and John Todd promotes shop painting with field finish coating. Scenarios are posed by Trimber, as Todd and Reardon take turns providing support to their cases while divulging plenty of information on the subject. This exemplum is based in part on the content of that podcast. 

 

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