Metal Preparation of Steel

Ask an Expert From: Products Finishing, , from Products Finishing magazine

Posted on: 10/25/2011

We are planning to powder coat the part with a TGIC polyester, but we do not have a system yet. Will it be acceptable to blast the surface and not add any chemical pretreatment? Do I need a primer coat under the TGIC polyester?

Q. We are designing a new product that will be fabricated of hot-rolled steel. The product will be used in outside installations, but the corrosion resistance is not critical, and we plan to offer a warranty of one year against coating failure related to corrosion. We are planning to powder coat the part with a TGIC polyester, but we do not have a system yet. Will it be acceptable to blast the surface and not add any chemical pretreatment? Do I need a primer coat under the TGIC polyester? L.A.

A. The simple answer to your first question is yes, you can powder coat over a blast-only treatment on hot-rolled steel, and you should be able to avoid corrosion in the first year if the blast surface is uniform and clean. There are other concerns that you should consider, though.

First, you will want to use a cutting type of media, probably an aluminum oxide, and make sure that you remove the organic soils and inorganic soils on the metal surface. You will want a minimum of at least 3 mils of powder over the entire surface. Create a profile in the steel with a measureable distance between the lowest point (the valley) and the highest point (the peak). You must apply enough thickness to provide the 3 mils minimum over the peak area of the part so your average film build will be 3 mils plus the peak-to-valley distance. Second, you cannot have any voids in the coating, so take care to get the needed coverage in recessed areas and on sharp edges. Test for voids in the coating using a pinhole detector. Handle the part carefully after coating to avoid mechanical damage.

You should also consider that outdoor use in a dry inland climate is considerably different than an installation near a salt-water sea coast. If the part is along the ocean edge, the corrosion warranty will be challenging, to say the least. If you are going into a more challenging environment, I would recommend a primer in addition to the polyester. Pay attention to these issues and you should be able do this without chemical pretreatment.
 


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