InProcess Quality Actions

Ground Coat. Parts rejected at the ground-coat stage are repaired and refired, if necessary, before the cover coat is applied. It is good practice to confine repair techniques to the defective areas. It is important to remove dirt particles, scale, and similar contaminants with a sharp-pointed instrument. The disturbed area around the site should be lightly stoned. The dust generated should be blown off. Ground-coat enamel is spotted-in at the repair area, and the entire piece is lightly dust coated. The piece is then dried and fired.

Lumps, handling defects, chips, and similar flaws in the ground coat are normally lightly stoned, and if this proves satisfactory, the parts are ready for cover coating. Otherwise they are treated as described above.

Defects that cover more extensive areas or are located at or near the steel enamel interface, such as burnoff, blisters, "copperheads," embedded grit particles, and dents, require different repair techniques; it is often necessary to grind down to bare steel. Grinding may be accomplished with a power grinder, using an 80-grit silicon carbide disk. A damp sponge placed under the work area prevents overheating. After firing, the repaired ground coat area should be hand stoned to blend it into the surrounding area of the surface.

Cover Coat. Defects that are missed during inspection of the ground coat usually become visible in the cover coat. Specks, blisters, copperheads, and dents are typical of such transmitted defects; they need to be removed by grinding into the ground coat. The ground coat is then repaired as described above, before another coat is applied. Cover-coat defects such as lumps, handling defects, chips, and thin coating areas are repaired by stoning, respraying lightly, and refiring the piece.

Direct-on cover-coat applications are repaired in much the same manner as ground coats. If the defect area is small, a light grinding or stoning is all that is required. This is followed by a localized application of cover coat and then a full dust coat, drying, and firing. If the repair requires grinding into the steel, then a ground coat patch in that area is required for adherence. A half coat of cover-coat enamel over the fired ground-coat patch is required, followed by a dust coat over the entire piece.

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