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Application Considerations

Type of Metallic Coating

All of the USS metallic-coated steels can be used to produce prepainted sheet: hot-dip galvanized, galvannealed, GALVALUME®, electrolytic zinc and zinc-iron alloy coated, and terne coated sheet.  As with unpainted products, a specific coating weight for the metallic coating should be selected. Even though the metallic coating is covered with a paint, the thickness of the metallic coating has a substantial influence on the performance in service. For example, perhaps the most common and easiest to understand is the influence of galvanized coating thickness on the tendency for undercutting corrosion of the paint at a scratch in the paint. A thicker galvanized coating slows the rate of paint-film delamination simply because the rate of dissolution of the zinc layer is slowed. G90 hot dip galvanized or AZ50 GALVALUME® sheet with a high quality paint system are perhaps the most commonly produced products in that they are the preferred products for use as metal building panels.

Properties of the Base Steel
U.S. Steel Prepainted Coated Sheet is available with essentially all but one of the steel product designations, ranging from CS Type B to DDS to High Strength steels. The one notable exception is the family of Dent Resistant BH steels. Processing through a paint line that includes curing at temperatures approaching 500°F can negatively impact the performance of the Dent Resistant grades.

Paint System
U.S. Steel Prepainted Coated Sheet is normally painted using a two-coat system, in which a primer is applied before the topcoat or finish coat. This is especially true for the side of the sheet that is considered the exposed side such as the outer surface on a metal building or appliance. Advantages of two-coat over one-coat systems are:  improved paint adherence, increased corrosion resistance, uniform film coverage and reduced pinholes or paint voids in the finish.  Also, a two-coat system allows more precise color match because the finish coat is applied onto a constant background color.

A two-coat system typically includes a primer that is approximately 0.2 mil thick.  The primer provides improved paint adhesion and corrosion resistance. The top coat provides the color and overall appearance desired for a specific application. Often, the top coat is about 0.8 mil thick.  It should be noted, however, that there are a wide variety of paint systems and film thicknesses.  Paints are selected for a number of reasons, including color, flexibility, hardness, gloss, resistance to staining, corrosion resistance and resistance to chalking and fading.  The potential combinations of primers, topcoats and film thicknesses are too numerous to detail for all industry applications.  They should be designed specifically for each application on the basis of the desired attributes. Thus, ordering of prepainted sheet products typically requires detailed dialogue with paint suppliers.

For most applications of prepainted sheet, a top coat is usually specified only for the exposed outside surfaces.  A thinner wash coat is often adequate for inside surfaces that are generally not visible in the final application.  For example, metal building panels have a top coat of paint on the exterior where best appearance and maximum corrosion resistance are desired.  A wash coat often is adequate for the reverse, unexposed sides of the panels.  However, if the inside of the building also requires superior corrosion resistance (e.g. a manufacturing process that emits corrosive fumes), a high quality top coat may be required for both sides of the panels.  As a minimum precaution, application of a wash coat is recommended for the reverse side to prevent marking of the finish coat from adjacent wraps in the coil and provide an overall higher quality product.

Paint thicknesses of 2 to 4 mils are used fairly frequently. Generally, these thicker paints provide added resistance to corrosion for special applications such as the interior of an industrial manufacturing shop that involves emission of corrosive fumes, or when a manufacturer wants to distinguish his product from competitors, i.e., offering a premium product.

It is possible to specify prepainted steel sheet product by performance attributes which are verified by quantitative standard tests. Such a specification may include these attributes: paint hardness, overall corrosion resistance, paint flexibility, paint adhesion, chalk and fade resistance, color match, gloss, solvent resistance, detergent resistance, abrasion resistance, and water resistance. When these items are clearly identified, a well-designed and engineered paint system can be applied to provide excellent performance in many applications.

The paint coating guide below indicates the relative paint film characteristics for four commonly used types of paint.  This guide, prepared by the National Coil Coating Association, is for general information only.

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