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If a flat strip of the ordinary sheet iron about 4 inches wide and 4 feet long is held at each end and bent in an attempt to form an arc, it will kink as shown left, and when released, the kink will tend to remain. In other words, this metal has very little life or temper.

. Soft metal when bent tends to kink	Body metal tends to form an arc instead of kinking when bent

A piece of body steel formed in the same manner will not kink but will bend in a continuous curve, as shown right, and will spring back to its original position upon release. This indicates the consistency of body steel and is mentioned here to emphasize that when a body or fender is dented, the metal is in a forced position, and if there are no sharp creases in the dented surface, the metal will spring back to its original position with just a little help of the right kind at the right place.

Body metal in its natural flat or slightly curved position, is more flexible and has more spring effect than that which has been forced into a shorter and sometimes compound curved position, such as the rear corner of a body or top which has been formed with dies which was caused by collision.

During this bending or stretching process the grain or structure of the metal is disturbed which causes it to have a little different characteristic. In other words, it is stiffer and has less spring effect.

A a door panel of which the entire surface was pushed in, but not far enough to cause a stretch, and no sharp creases were made in the dented surface. By lifting the upholstery to allow access to the back of the panel and then exerting a little pressure at one edge of the dent, the entire dented surface will spring out similar to the action of the bottom of an oil can when removing the thumb after pressing it. Only slight hammer and dolly work may be necessary around the rim of the dent.

Frequently the dented surface will be sharply creased in one or more places by striking or scraping some sharp object during the accident. The metal may or may not be stretched at these creases ; this will be determined after the usual smoothing processes.

These creases will form stiffening ribs in the metal and prevent it from being easily returned to its original contour. However, pressure will restore a portion of the original shape quickly, so that the creases may then be treated with hammers and dolly blocks. If the dent does not come out by the application of a reasonable amount of simple pressure, such as a push with the foot in the case of panels, or use of a C clamp on fenders or crowned surfaces, then the work is begun with the hammer, which operation is known as "roughing out," and is fully described later in this chapter. Care must be exercised in this roughing out operation to avoid hammering the metal out beyond its original curve as this stretches it.

Body metal can also be stretched by continually striking it with a hammer while holding a dolly directly underneath the point of the hammer blow. It is necessary at times to be able to stretch this metal as well as to shrink it. Although this does not happen often, it is well to study and practice it. Frequently a beginner will stretch the metal unintentionally while trying to smooth out a dent. This difficulty can be overcome by practice.

While discussing stretched metal, a word of caution might be inserted here. Stretched metal requires shrinking and since shrinking is done with heat, it is well to remember that metal loses its temper or spring effect when heated to a cherry red. Therefore, it cannot be too strongly emphasized that heat should only be used when and where absolutely necessary.

Bumping and dinging

Hammer and dolly work is the basic operation in refinishing bodies and fenders. This operation is the oldest part of sheet metal working, and is the nucleus around which other operations are built and elaborated to broaden this work into a complete service.

Hammer and dolly work may be conveniently divided into two broad classes : roughing and finishing, or as more technically known, bumping and dinging.

Bumping.—Bumping is the first step in hammer and dolly work. It is that of using one of the heavier or specially designed hammers, to work out the larger or sharply creased dents while roughly shaping the metal back to approximately its original contour. The remaining smaller dents are left for the finishing and dinging operation, which is explained later.

Study carefully each job before the work is actually started to discover the most effective and simple way of working out the damaged surface. Remember, a few careless blows may require a lot of additional time to restore the damaged portion to its proper level.

No specific instructions can be given in this work, as there are no two dents alike, and each dent may require just a little different procedure depending upon condition and location.

Bumping is accomplished by working from the opposite side from which the dent was made. As a general rule, on badly damaged surfaces, the hammering is begun on the sharpest creases or deepest dents, holding a heavy dolly block on the rim of the dent. On broad shallow dents it is started at the edge, working toward the centre. The object during this procedure is to always remove the greatest strain first with the least amount of hammering.

On large surfaces, such as door and body panels, after the upholstery has been removed, the dents can often be roughed out to a great extent by exerting pressure with the foot, body jack, or large "C" clamps.

When the dent occurs directly over a fender bracket, body support, or in places too narrow to allow the use of a hammer, then one of the fender or body spoons can be used to pry the metal out, and later, as an anvil, while dinging.

Care must be exercised in this operation to avoid hammering or forcing the metal out beyond its original contour, as this stretches it. There are times, however, when the force of the blow that caused the dent was so great that the metal was stretched at the time of the accident. This condition, and the extent thereof, can best be determined after the usual bumping and dinging process is completed.

On badly damaged surfaces, after they are roughed out, it is sometimes advisable to rough ding them with the use of a wood, fibre, or rubber mallet and dolly. The mallet eliminates the danger of stretching the metal.

Dinging.—The dinging operation is that of smoothing out the smaller dents remaining after the usual roughing out has been done. The principle is simply that of working metal on an anvil with a hammer, the anvil being the dolly block. Figure left illustrates the dinging operation in its simplest form. Note carefully the position of the hammer in relation to the dolly block.

Before starting any hammer and dolly work, clean thoroughly, removing all mud, tar, grease, etc., from both sides of the metal. Use a scraper to clean the underside of fenders or inside of door or body panels. Tar can be removed from a lacquered surface without marring it by soaking it with kerosene oil or lard until dissolved. Any dirt sticking to the metal will scar the surface of the dolly or hammer. Using a scarred dolly or hammer will cause the body metal to become pitted, requiring more hammering and filing to secure a smooth finished surface.

A thin coat of motor oil on the painted surface of a fender or body panel will prevent marring it when striking with the hammer, and the gloss of the oil will tend to show up the defects in the surface, especially where the paint has become dull.

For the best and quickest results, place the job in a position so that the reflection of the light shows up the high and low spots.

The dinging operation consists principally of repeatedly placing the dolly, as shown above, on the remaining low spots, and applying outward pressure, at the same time striking lightly with the hammer on the high spot or rim of the dent. The kind of dolly to use is governed by the location of the dent. This procedure is continued until the surface appears to be smooth.

Rubbing the hand lightly over the surface will reveal high or low places that are not noticeable to the eye. After dinging the metal as far as the sight or touch will permit, the body worker's file is brought into use. Drawing the file over the surface a few times will reveal any remaining defects which were not noticeable by the sight or touch, and which must be smoothed up with the hammer and dolly. Repeat this filing and dinging until there are no dark spots remaining; this will indicate that the surface is uniform. Use of a cloth or canvas glove on the hand will accentuate the "feel" of high and low spots.

Don't file away wrinkles or high spots. Remember the metal is not thick. Use the file only for showing up high and low spots ; then use the hammer and dolly to level them out. To complete the job the surface should be sanded to remove any rough nibs or scratches usually caused by filing. This is easily done if a portable electric sander is available, using a disk of No. 60 or 80 grit. If not, the smoothing can be done by hand with a piece of medium fine emery cloth, or sandpaper.

Remember that a coat of lacquer or enamel will not cover or hide any defects, but instead tends to magnify them.