Painting Faying Surfaces

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Author Message Kenneth C. Crocco Senior Member Username: kcrocco Post Number: 119 Registered: 04-2003 Posted on Wednesday, November 28, 2007 - 03:41 pm:    How do you specify painting exterior architecturally exposed structural steel? Particularly the "faying" surfaces. We currently have a specification that requires two coats of primer for surfaces that are not accessible after installation. These surfaces don't receive a finish coat. I am confident this works for interior AESS, but what about exterior? Will the space between the surfaces eventually get wet and corrode? I looked in AISC Section 10 for AESS and painting is not addressed there. Ralph Liebing, RA, CSI Senior Member Username: rliebing Post Number: 743 Registered: 02-2003 Posted on Wednesday, November 28, 2007 - 03:52 pm:    I have a great paint/coating resource in the form of a corrosion engineer at Sherwin-Williams. Very knowedgable and helpful. Might try him tippoliti@sherwin.com Mark Gilligan SE, CSI Senior Member Username: mark_gilligan Post Number: 8 Registered: 10-2007 Posted on Thursday, November 29, 2007 - 04:04 am:    Consult with your structural engineer. If he is using high strength bolts in a slip critical application the application of the wrong type of paint to the faying surface may invalidate his design. I am dubious that this does any good in the exterior condition. Possibly the SSPC has some guidance. In the case of interior steel unless it is visible or the steel is subject to significant humidity or other corrosive conditions I would not paint it. AISC recommends no shop coat for steel not exposed to view and in conditioned spaces. Roger Tegtmeyer (Unregistered Guest) Unregistered guest Posted on Wednesday, November 28, 2007 - 10:26 pm:    At the risk of revealing the obvious, bolted structural steel connections are designed in part based on the tendency of the surfaces in contact to move (twist) under load. The 'slipperier' the connection, the more bolts and/or gusset plates to resist that moment. Conversely, the more 'grippier' the connection, the fewer bolts required and gusset plates are smaller or not required. AISC classifies these faying surface connections as either Class A or Class B. This rating is a result of a test of the slip resistance of the bolted surfaces and a 30 day creep test. Class A slip has a slip coeficient of .33 or higher. Class B slip has a slip coeficient of .50 or higher. Thus Class A requires more bolt holes, more bolts, etc and is more costly for the fabricator; Class B is preferred because of the reduced cost. Typically, bare steel (obviously rusty steel) is a Class B connection. Many organic coatings may meet the Class A standard. Class B connections are provided by specific zinc rich coatings designed to meet that standard. These are typically inorganic zinc rich primers and organic zinc rich primers with a high level of zinc loading. Hot dip galvanizing per ASTM A123 is also said to be a Class B slip though the wide variance in the roughness or smoothness of hot dip galvanizing would cast some doubt of the real life probability of a Class B rating but officially it is. Consult the American Hot Dip Galvanizers Association for more information. So, high performance metallic zinc-rich primers and hot dip galvanizing can provide the optimum slip coeficient rating. These same materials solve the problem or concern about rust in the connection, concern about deterioration of the connections slip resistance and subsequent unsightly rust stain bleeding from connections in weathering environments. I recommend that organic zinc rich primers or inorganic zinc rich primers with a Class B rating be utilized for these connections for cost reduction, long term performance and to provide long term corrosion protection for these connections. Bolted connectons utilizing zinc rich primers are typically used for all types of industrial applications including nearly all steel bridges, power plants, and refineries to mention a few. I recommend that these surfaces be abrasive blasted to a SSPC SP 6 Commercial Blast Finish and 1-3 mil blast profile. At a minimum (as opposed to coating the entire beam or column) the faying surfaces should be primed with a single coat of a Class B slip coefficient zinc rich primer at its recommended dry film thickness. This surface should be masked so that no topcoat is applied. Typically the 'exterior' surfaces of these bolted connections, the bolt head on one side and bolt with nut area, should also be masked. After the connections are bolted together, these 'external surfaces' should be cleaned of any residues or other bolting lubricants and touched up with a surface tolerant primer, usually a catalyzed epoxy or phenalkamine epoxy mastic primer, and then topcoated if desired. I regret the length of my response but I hope I have addressed the issue adequately. However, I welcome any additional questions. Anonymous   Posted on Wednesday, November 28, 2007 - 04:33 pm:    Just be aware that faying surfaces of slip-critical connections, if painted, requires primer that has been tested and classified for slip-critical connections. If exterior corrosion is an issue and galvanizing is not an option, you might want to consider zinc primer...again, if slip-critical connection, tested and classified accordingly. Tom Heineman (Unregistered Guest) Unregistered guest Posted on Friday, November 30, 2007 - 01:51 pm:    Why not a sheet of pure zinc foil, neatly trimmed at the edges? Roger Tegtmeyer (Unregistered Guest) Unregistered guest Posted on Friday, November 30, 2007 - 04:25 pm:    Other than bare steel faying surfaces which are considered Class B, coatings for faying surfaces are tested by an independent test lab to very specific test protocol and the data sheets for coatings that pass either Class A or Class B will clearly indicate that product feature. If the product data sheet does not provide that information, it has not been tested or passed that test critiria. There are many such coatings/primers that are so certified and I recommend that those products be specified. And as a previous responder has written, that is an important issue to the structural engineer and he will have designed the connections around Class A or B slip critical connections--or not use bolted connections at all.