Painting of galvanized steel

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Author Message Ellis C. Whitby, AIA, PE, CSI, LEED® AP Senior Member Username: ecwhitby Post Number: 48 Registered: 03-2003 Posted on Thursday, June 12, 2008 - 09:39 am:    One of our clients has stated that we are wasting money by specifying painting of exterior galvanized steel. I have been under the impression that if properly prepared, painted galvanized steel is no more likely to peal than painted steel. Am I wrong? My PM has also asked that I provide “technical justification” for my recommendation that painted galvanized steel will last longer, with less maintenance, that either painted steel or unpainted galvanized steel. Any sugguestions? My justification has been that the paint is never maintained as regularly as it should be (hence the galvanization). I also state that the paint protects the galvanization while the paint is intact. The project is a commercial facility in an inland location. Edward R. Heinen, CSI, CCS, LEED AP Senior Member Username: edwardheinen Post Number: 14 Registered: 04-2006 Posted on Thursday, June 12, 2008 - 09:50 am:    The American Galvanizer's Assoc. (www.galvanizeit.org) offers a free download publication, Duplex Systems: Painting Over Hot Dip Galvanized Steel (2005), which discusses reasons for and benefits of painting galvanized steel. J. Peter Jordan Senior Member Username: jpjordan Post Number: 273 Registered: 05-2004 Posted on Thursday, June 12, 2008 - 10:31 am:    You did not mention which island; there are a variety of exposures depending on location; however, ... Good, hot-dipped galvanized steel should last indefinitely, the zinc eventually ozidizing to a protective coating. In such a case, painting would be cosmetic. In severe exposures with prevalent winds blowing inland from over salt water, galvanizing probably works almost as well as Type 316 stainless steel (some may disagree with me). Go to a seaside location and look at the metal street light poles. Unless they are decorative (real or faux cast iron with a paint finish). they are probably galvanized steel. No one does any maintenance to these suckers, and they will last 10 to 20 years unless the exposure is extremely severe. I have recently finished specifying an industral building located in a port area just off Galveston Bay. The structural components are standard metal building system components and the finish is hot-dipped galvanized steel. I have seen other instances of similar applications in this area. There are some caveats you should remind yourself of. Caveat No. 1: The zinc coating can be damanged during installation through normal abrasion and welding. While very minor damage to the zinc coating may "heal", the damage may be enough to cause rust. Zinc-rich "galvanizing repair" paints are supposed to be used to coat the damaged areas, but I would suggest that the zinc in such paints simply does not bond to the underlying steel in the same way that molten zinc does. Caveat No. 2: There are a variety of zinc coatings and hot-dipped should be the only one accepted under these circumstances. That being said, the quality control over the amount of zinc deposited is poor which results in uneven applications (possibly even coatings that do not meet standard specifications). A high-performance coating rep has told me that he would prefer to specify a zinc-rich primer on properly prepared bare metal rather than a primer for galvanized steel because he really doesn't know what he will get from the shop. Caveat No. 3: Different components in an assembly may have different standards for galvanizing. If someone get fasteners with an electroplated zinc coating, these will not have the same life as adjacent hot-dipped galvanized items. The resulting corrosion will probably impact the performance of all components causing what may seem like premature failure. A coating system over the galvanized substrate along with regular maintenance will almost certainly prolong the useful life of the assembly. How long depends on the quality of the galvanizing, surface preparation, and the coating system specified and the diligence of the maintenance. Look at the installation required. It is best if you can maximize shop fabrication with entire assemblies galvanized after fabrication. Shop application of coatings is also preferred. All of this will have to be touched up (or "repaired") in the field after installation, but you can be better assured of proper preparation of substates (the key to a good coating system) if the coating is shop applied. If you decide to rely on the paint finish to protect the steel, use a high performance system with a good zinc-rich primer (this is not the stuff you get at Home Depot), and it may be more expensive than specifying galvanizing. A high-performance coating system may, in fact, be a better choice than galvanizing in some circumstances. Specify proper preparation for the substrate and then cross your fingers. Any damage to the coating that gets through to the steel (intentional or otherwise) will probably result in a coating "failure" due to corrosion of the the underlying substrate. At the end of the day, the visual quality of a painted finish may be preferred. If the paint system is conceived of and specified as more of a "decorative" finish, you may be specifying a maintenance issue for your client. A good high performance coating system can work with the galvanized substrates to improve the protection of the underlying substrate, reducing required maintenance over the life of the building. Anne Whitacre, FCSI CCS Senior Member Username: awhitacre Post Number: 783 Registered: 07-2002 Posted on Thursday, June 12, 2008 - 01:58 pm:    I've also specified non-galvanized, but high performance finishes that require specific preparation and shop application of primer, with the finish coat to be field applied. (Tnemec is one series of systems I've used for this). Many of the pieces of steel I've had on projects are either too big for hot dipped galvanizing, or too small to be hot-dipped, and we've had to use other systems. I've done very well with high-performance coating systems in coastal environments, both in the southeast and in the northwest. At the moment, I'm using high performance coatings in both Miami Beach (coastal) and Las Vegas (hot, scouring winds with sand). I agree with the other writers that hot dipped galvanizing isn't enough -- in areas where welding is to take place, the galvanizing is often not repaired properly, and fasteners and fastener holes can be a problem. Jerry Tims Senior Member Username: jtims Post Number: 20 Registered: 04-2005 Posted on Thursday, June 12, 2008 - 04:15 pm:    Just to add to what Anne said.....if you'll be using a high performance coating....like one of the Tnemec products, be sure and check the paint manufacturer's surface prep requirements. Much of the time, the manufacturer requires the steel to be shot blast cleaned, which makes it a waste to specify galanized since the prep work will remove the galvanized coating anyway. If the steel is to receive a standard (non-high performance) paint coating, we always specify that exterior steel be galvanized. Ellis C. Whitby, AIA, PE, CSI, LEED® AP Senior Member Username: ecwhitby Post Number: 49 Registered: 03-2003 Posted on Thursday, June 12, 2008 - 05:41 pm:    Thanks for the responses. Edward, teh reference to www.galvanizeit.org was very useful. We still haven't convinced the Owner, so we probably will keep the galvanization and delete the paint. The site is inland so we should still get 30 plus years before major maintenace is required. John Bunzick, CCS, CCCA, LEED AP Senior Member Username: bunzick Post Number: 912 Registered: 03-2002 Posted on Sunday, June 15, 2008 - 12:27 pm:    No way will proper preparation remove the galvanizing. When painting galvanized steel, we typically request the primer (at least) to be applied in the galvanizer's shop, or within 12 hours of galvanizing. This gets the primer on before the formation of initial zinc oxide, which is the main inhibitor to the adhesion of paint. While hot dipped galvanizing is a long-lived coating, I've seen situations where it has just eroded/corroded away, leaving the steel to rust. Thus, for the longest lasting finish, we specify both. (Or, where aesthetics is an issue.) On the other hand, the providers of high-performance coatings tell me that with near-white-blast prep, and a properly selected system of high performance coatings, the life will equal or exceed hot dip. I have no data to support this, but the concept is interesting. Philip R. Carpenter AIA Senior Member Username: philip_carpenter Post Number: 8 Registered: 08-2008 Posted on Thursday, August 21, 2008 - 08:07 pm:    A bit off the subject, but close: A few years ago a firm I was with designed a steel canopy system for a site drop-off - all very fancy typical architecturally designed type of stuff - the entire assembly was hot dip galvanized and I was shocked at the visual distortions and twisting that the contractor insisted was "the nature of the galvanizing - nothing we can do about it". Fortunately the Owner was not so picky as I was and the problem resolved itself - but I have been left with a bit of apprehension about the visual aspects. I see hot dip galvanized assemblies all the time that are not twisted, distorted etc and notice an ASTM about this problem. I would like to know more about how to properly specify hdg assemblies to ensure the next Owner will not be after my head nor will I be embarrassed. Simply citing the ASTM is not enough - I like to know what I am specifying. Any help would be appreciated. William C. Pegues, FCSI, CCS Senior Member Username: wpegues Post Number: 747 Registered: 10-2002 Posted on Friday, August 22, 2008 - 12:32 am:    Philip, This happens. And, it is difficult to get a technical grasp on without someone with first hand experience. About 2 years ago, I was working with an interior designer that wanted a really industrial look and fell in love with the look of galvanized coatings. Naturally it is all very different depending on whether it started out as a coil stock or was a formed or cast or other type of shape as to how the galvanizing actually 'looked'. So i got with a really good local galvanizer. Fortunately, my this was just after a our CSI region conference where we had had a really good galvanizer give a general presentation on what and how they do this. So i called him, and we talked about thing like warpage, and the major point, how to achieve as much as possible a uniform finish. Essentially, I was developing an architecturally exposed galvanized finish spec. It all came to naught as the designer changed their mind, but, what I gained from it was that if you can find a good galvanizer who will talk with you, it won't take more than 20 to 30 minutes and perhaps a couple exchanges of FAXes to get exactly what you need. A lot of the warpage problem can be taken care of by specifically requiring different kinds of support on the dipping rig. Say nothing, you get what comes out. But, I would have no idea what to say without talking to a galvanizer and explaining my concerns. Its all solvable. William Philip R. Carpenter AIA Senior Member Username: philip_carpenter Post Number: 10 Registered: 08-2008 Posted on Friday, August 22, 2008 - 05:46 pm:    i posted a similar message about the warp issue to the galvanizeit website and they responded with some information that i could share if anyone wants. dont know how or if it is possible to link such documents on this site. Joseph Berchenko Senior Member Username: josephberchenko Post Number: 16 Registered: 08-2003 Posted on Monday, August 25, 2008 - 02:11 pm:    Philip - Could you post the web site url to the galvanizers discussion? Would be interested to hear what galvanizers have to say. Philip R. Carpenter AIA Senior Member Username: philip_carpenter Post Number: 11 Registered: 08-2008 Posted on Monday, August 25, 2008 - 09:01 pm:    Here is the content of their reply but i have no clue as to how to forward the 3 PDF files they attached to their response to me (sorry!) Their reply: I have attached several publications that give design recommendations for controlling distortion when galvanizing. An ASTM specification exists for controlling distortion - ASTM A384. This specification can be purchased from ASTM directly for $35. Alternatively you can purchase it from the AGA for the same price ($35), but the AGA specification package includes seven other galvanizing specifications for free. Let me know if you'd like me to reserve a copy for you. Bernardo Duran Corrosion Engineer American Galvanizers Association "Protecting Steel for Generations" bduran@galvanizeit.org Phone: 720.554.0900 Ext. 21  Fax: 720.554.0909 www.galvanizeit.org © 2008 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.