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Chapter 8 Nonstructural Panel Repair
grains to thicken. The thickening causes a reduction in
surface area, thereby eliminating the stretch.
The amount of metal shrinkage that occurs depends
on the size of the heated area and the heating tempera-
ture. The heating process must be carefully controlled.
Stretch damage most commonly occurs on low-crown
panels. Uncontrolled use of heat on a low-crown panel
will cause warp damage. Warp damage is a group of
high and low areas in a panel. Warp damage is difficult
to remove, if not impossible. Always use the minimum
amount of heat required when shrinking.
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 avail-
able, simply 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 proce-
dure. 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).
Simply 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 8-34. 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 accessible, 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 rustproofing 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
Figure 8-33. The first step in this repair is to align the
door edge with locking pliers. Tapping on the ridge
while pulling the pliers will move the edge.
Oxyacetylene torch
Figure 8-34. A torch is used to heat a small circle on a
stretched panel. Notice the color of the heated metal.