189 Chapter 9 Nonstructural Panel Repair Copyright Goodheart-Willcox Co., Inc. One heat shrinking technique commonly used to eliminate false stretch involves the use of a draw pin welder and a shrinking tip. If a shrinking tip is not available, use the draw pin welder without a pin. If hammering on a nearby body line does not cure false stretch, heating with the draw pin welder will remove it. Locate the false stretch and pop it in. Grind or sand off the paint from the false stretch area. Push the shrinking tip onto the false stretch and pull the trigger on the draw pin welder. Keep the welder on for about one second. The false stretch should remain down and not pop up. If it does pop up, repeat the procedure. If the false stretch is so big that pushing in on one area causes another area to pop up, try shrinking the metal between the two areas. Usually, a false stretch area that has been shrunk must be filled. You can also shrink a high spot using a draw pin welder and the draw pin welder tip (or a shrinking tip).Place the tip on the high spot and press the trigger in for one second. Then tap the heated area twice with a flat-faced dinging hammer and immediately cool the heated area with a wet rag. This method works well on high spots that result from pulling draw pins above the surrounding metal. Another shrinking method involves the use of an oxyacetylene torch or a butane-fueled micro torch to remove a low spot. This procedure will require heating, putting down the torch, and using the hammer and dolly, all in a brief period of time. Therefore, a dinging hammer, a flat dolly, and a wet rag should be nearby. Be sure to locate a safe place for the hot torch before you start. The stretched area should be stripped to bare metal first. When using an oxyacetylene torch, use a small tip, such as a #1 tip. Light the torch and adjust it to a neutral flame. The hot spot of the flame should be concentrated on the deepest part of the stretch. The heated area must be no bigger than a dime. See Figure 9-34. As steel heats up, its color changes from silver to blue and then to red. The heating will cause the grains to thicken, reducing the surface area. With less surface area, the metal contracts, raising the low spot. Immediately after the steel changes color from blue to red, turn off the torch and set it down. Tap around the heated area with the dinging hammer. If the back side of the panel is acces- sible, the dolly can be used to support the panel. Use light hammer-off-dolly blows to level the area. Once the color has disappeared, the wet rag can be used to cool the area. Using the wet rag to cool the area while the metal is too hot with color can make the metal brittle. The area that has been shrunk will be harder than the surrounding metal. If many shrinks are needed, shrink the most stretched area first, but do not quench it (cool it with the wet rag). Shrinking with heat will burn rust- proofing from the back side of the panel. Be sure to replace the rustproofing after heat shrinking. As damaged metal is straightened, a stretched area, originally a low area, may become raised. The area is still stretched, but it is now high instead of low. A torch can also be used to shrink a stretched high spot. Locate the highest part of the stretch. This area will have the most stretch. Shrink this area first. Heat up the area with the most stretch until it just reaches a red color. The heated portion should be no larger than a dime. Put the flat dolly on the back side of the panel. Use light off-dolly taps around the heated area. Goodheart-Willcox Publisher Figure 9-34. Bring the hottest part of the neutral flame down on the high spot. Make a small circle. As soon as the color changes from blue to red, remove the flame and tap around the heated area. The Xs mark where to tap. High spot X X X X X X X X Goodheart-Willcox Publisher Figure 9-33. The first step in this repair is to align the door edge. Tapping on the ridge with a dolly will move the edge. Dolly