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volume concentration is surpassed. This means there hardly is enough liquid (binder) to provide flow or surface wetting. The very strong adhesive and cohesive properties are achieved by very complex chemical bonding of zinc, silicate, and steel. So what happens when you recoat a (fresh) IOZ layer with the same material? The liquid part gets sucked into the porous IOZ layer and is not available to react with the newly applied material. This layer will not get any cohesive strength and remains powdery. This is not a good substrate to continue with the succeeding coats of the coating system. Some suppliers recommend heavily thinning down the IOZ when you apply additional thickness. They recommended this in order to have more liquid avail- able for saturating the first layer. They also limit the time that you can still do the recoat. My approach is this: When the IOZ overall is somewhat below the dry film thickness (DFT) in the specification and it is used as a system primer, leave it as it is and compensate with sealer or inter- mediate coat to achieve the total system DFT. But do this only if you have the authority to do so. When IOZ is used as a single- coat system, recoat with thinned-down material if you are still within the manufacturer's recommended limit. Otherwise, reblast and recoat. Masking Off Galvanizing Q Does anyone have any experience with preventing galvanizing from adhering to surfaces? We have a job coming up that requires bridge beams to be hot-dip galvanized, but the top flanges must have no galvanizing on them. This is to allow welding of grating and shear connectors. Some advice we have received is to apply "two coats of a good epoxy and the coating will scrape off." Frankly, I'm skeptical. A The advice given is correct. At the galvanizing temperature, epoxy would decompose to form a carbon. Write In Write in Reader Inquiry #266 January 2009 J www.coatingspromag.com 17 Carbon paste is a very effective means of preventing zinc adhesion. Alternatively, water glass (sodium silicate) also would act as a stop-off. After applying the stop-off, you would have to remove both galvanizing and stop-off residue before the zinc-rich primer could be applied. Otherwise it probably will fall off as well. A Using an epoxy may be valid. My experience is that any paint left on the steel will not be removed during the pickling process, unless the steel is left in the pickling tanks for an extended period of time. Even then, the paint is not always removed. If the epoxy was confined to the area you want to mask only and the steel was left in the pickling tank only for the period of time required to prepare the bare steel, it very well may come out of the galvanizing tank with the flange uncoated or coated but not bonded to the steel. This is because the remain- ing epoxy would prevent the molten zinc from wetting out the surface, thus preventing the normal coating reactions. In the American Hot Dip Galvanizers Association's Inspection Hand Book, one of the causes given for bare spots is paint on the steel surface. A A Instead of using an epoxy masking coating, you may just as easily apply a graphite-rich (carbon) coating to the flanges — and the cost wouldn't be as great. Here in the U.K. we sometimes mask off small areas of galvanizing with heat-resistant sticky tape. You stick it on before dipping and then peel it off afterwards. It works well for small areas but it might be more of a pain with whole flanges. CP