690 Section 5 Refinishing Copyright Goodheart-Willcox Co., Inc. travel speed with more overlap. Clearcoat is applied as a full wet coat. A drawback of applying too wet is the risk of trapping solvents. Drop Coat Some paint manufacturers recommend a drop coat be sprayed once the basecoat has covered the under- coat. A drop coat is the last coat of basecoat. A drop coat will set the metallic and prevent mottling. Mottling occurs when the aluminum flakes in a metallic color are not uniformly distributed. In mottling, the applied paint has light and dark bands or spots because the spray gun is not applying a uniform amount of paint. Typically, mottling is caused by a dirty air cap, or the spray gun is not held perpendicular to the surface. Usually, a drop coat is sprayed at a lower than recom- mended air pressure setting—usually 25 psi for a conventional spray gun and 5–7 psi for an HVLP spray gun. All other aspects of spraying remain the same. Mist Coat Another method used to prevent mottling is a mist coat. A mist coat is the last coat of a metallic paint and is sprayed at twice the normal distance from the surface. This coat is sprayed onto the last coat of wet paint. The mist coat will settle on and melt into the wet paint. Tack Coat A tack coat is another type of dry coat. This coat is sprayed first, before other coats. The tack coat makes a rough surface so the next coats can adhere to it and is often used before spraying a full wet coat of single- stage. A tack coat can be applied by reducing the amount of material, increasing distance, or by moving faster. Blend Coat A blend coat is an over reduced (30–50%) basecoat sprayed at one-half normal air pressure. The blend coat evens out metallic distribution to prevent mottling and makes a blend invisible. The blend coat is sprayed as the last coat of basecoat. Control Coat A control coat is used to uniformly orientate the metallic in a waterborne basecoat. Most metallic water- borne basecoats look mottled. Apply the control coat after the last coat of basecoat has flashed and has been inspected for coverage. On horizontal surfaces hold the spray gun at one and one-half the normal distance, move faster than medium wet coat, and follow a 75% overlap. On vertical surfaces hold the spray gun at twice the normal distance, move faster than a medium wet coat, and follow a 75% overlap. A solid color does not require a control coat. Dust Coat A dust coat will hide a waterborne basecoat blend edge. Use a C-stroke. For metallic basecoat, apply at one and one-half times the normal distance. For solid colors, apply at the normal distance. Paint Type All paints are not sprayed in the same way. Some paints, such as dark basecoats, are easy to spray. The paint covers the undercoat in one or two coats without mottling. Other paints, such as white solvent borne basecoats, may be more difficult to spray because of a tendency to run. Light colored metallics, such as silver, may easily mottle. Basecoats should be applied for coverage each coat applied as smoothly as possible. Clearcoat can be difficult to spray, especially a fast dry clear. The first coat of clear will brighten the basecoat so it is easy to see how the clearcoat is flowing out. The remaining coats of clear will be applied over a shiny clearcoat, making it difficult to judge the application tech- nique. Be careful on second or third coats of clear. Follow the established pattern and rhythm from the previous coats. Waterborne basecoats will have a specific appli- cation procedure depending on the brand of paint. This procedure can range from one quick coat immediately followed with a heavy coat to three coats applied in the same way as solvent borne basecoat. Waterborne metallic or pearl basecoats will look mottled as they are applied, even after the control coat. After the flash time has passed the basecoat application will look better. Spraying Body Panels The basics of spray gun handling can be applied to refinish any panel on a vehicle. This section is broken down into two parts—preliminary information and specific panel refinishing. Always keep in mind that spraying has a rhythm and the painter will establish this rhythm on the first coat of a panel. By maintaining a rhythm, the painter can apply additional coats of basecoat and clearcoat with satisfactory results. Spray painting is often referred to as an art. In this art, the painter masters all the variables—spray gun adjust- ments, spray technique, and paint type.