Mixing and Blending a Silver Metallic Bumper
A 2020 compact SUV needs its rear bumper cover refinished in a silver metallic. The adjacent left quarter panel is undamaged, but the colour on the bumper has a slightly lighter face than the quarter panel, and the shop's colour system shows two variants for the code. The job card requires 300 mL of sprayable basecoat. The product data sheet states 2 parts base to 1 part hardener by volume, then reducer equal to 10 percent of the combined base-plus-hardener volume. The technician mixes 200 mL base, 100 mL hardener and 20 mL reducer, sets a 1.4 mm nozzle gun to 2.5 bar inlet pressure, and plans to spray only the bumper. Shop conditions are 30 degrees C and 65 percent relative humidity. A test card sprayed at 20 cm distance shows a mottled, light-dark metallic pattern. The technician proposes adding more reducer and spraying a wet coat to make it flow.
1. Calculate the correct mix for 300 mL and identify the technician's mixing errors.
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Let the hardener volume be x. The base is 2x and the reducer is 10 percent of the combined base plus hardener, so reducer equals 0.10 times 3x, or 0.3x. Total volume is 2x plus x plus 0.3x, which equals 3.3x. For 300 mL, x equals 300 divided by 3.3, or about 90.9 mL. The correct mix is therefore about 181.8 mL base, 90.9 mL hardener and 27.3 mL reducer. The technician mixed 200 mL base, 100 mL hardener and 20 mL reducer, giving 320 mL total. The base-to-hardener ratio is correct, but the batch is too large and the reducer is only 20 mL instead of about 30 mL for 300 mL of base plus hardener. Adding more reducer and spraying wet is not a correction; excess reducer can cause slow drying, solvent pop and metallic floating, while a wet coat can worsen mottling.
2. Explain the cause of the mottled test card and state the correct gun setup and application technique.
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Mottling in a metallic basecoat is usually caused by uneven film build, incorrect air pressure, too much distance, poor overlap or a wet coat that allows the aluminium flakes to float and cluster. A 2.5 bar inlet pressure is likely too high for the gun and product; the technician should follow the data sheet, often around 2.0 bar for an HVLP gun, and adjust fluid and fan on a test card. The 1.4 mm nozzle is reasonable for basecoat, but the gun should be cleaned, the fan set for a suitable pattern, and the distance kept at about 15 to 20 cm with 50 percent overlap. Apply medium wet coats, not heavy wet coats, and use the recommended flash time. For metallic, a drop coat at reduced pressure and greater distance can help orient the flakes uniformly. The technician should practise on a test panel until the pattern is even before touching the bumper.
3. How should colour variant selection, blending and record keeping be handled?
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The technician should spray test cards with both variants over the recommended primer or sealer and compare them with the quarter panel in daylight, not under workshop fluorescent light. The variant that matches the adjacent panel, not just the old bumper, should be chosen. Because the bumper is plastic and the quarter panel is metal, and because metallic orientation changes with application, the basecoat should be blended into the quarter panel rather than stopped at the bumper edge. The blend area should be prepared and a blending solvent used if the system requires it. The job card should record the colour code, variant, paint batch numbers, reducer and hardener, gun settings, temperature and humidity, and the blend area. This record is essential if the customer returns with a colour complaint and allows another technician to repeat the match.