Indirect hammering (off the

Dolly block)

Indirect hammering is another technique which

uses hammer and dolly to level a panel surface.

A low area can be raised by hammering round

the outer edges in such a manner that the rebound

action of the block tends to push the low area

upwards to its original contour. This in fact is

achieved by the sequence of hammering just off, or

at the side of, the dolly block; hence the name of

indirect hammering (Figure 13.36). This technique

is used in conjunction with direct hammering or

planishing to achieve a final finish on the panel

surface. Metal that has not been excessively hammered,

displaced or stretched will have a tendency

Figure 13.34Using a high-crowned dolly

Figure 13.35Using a low-crowned dolly

Figure 13.36The technique of indirect hammering

Direct hammering requires skill in directing the

hammer blows and close observation of what you

are doing so as not to hit the metal too hard,

thereby displacing it. Perfect coordination between

your two hands is necessary to enable you to move

the dolly around under the damaged area and still

continue to hit squarely over it with the hammer.

Start hammering by using light blows; these will

not do the job, but will show you whether or not

you are hitting squarely over the dolly. Do not

forget to let the dolly just lie in your hand and to

grip the hammer loosely. A true ring will be heard

if you are directly over the dolly otherwise the

sound will be dull. Increase the force of the blow

gradually until you have found just the right force

Craft techniques and minor accident damage 361

to return to its original contour of its own accord.

This is due to the internal strain imparted to the

metal by the forming dies in manufacture. If the

metal is prevented from springing back by other

strains imparted to it by additional bends or creases

that have been formed by accident, the metal can

then be restored to its normal contour by relieving

whatever new strain is holding it out of position.

In direct hammering a dolly block having the

correct contour to match the original shape of the

panel is held under the low spot, and a series of

light blows are aimed around the outer edge of this

low spot, and slightly off the dolly block. The light

blows will not displace the surrounding undamaged

area, but the force of the downward blow will

be transferred to the dolly block. As a result of

receiving the hammer blows indirectly, the dolly

will rebound and the hand holding the block will

automatically bring it back in place so that it

imparts a light push upwards on the area. The

centre of the damaged area will slowly rise until

the original contour is restored.

Spring hammering

This is another technique of using hand tools to

smooth and level a panel surface. In this case only

a hammer is used, and it is not supported with

a dolly block. The technique is used to reduce

high spots which sometimes form as a panel is

planished. In some cases these high spots can be

reduced by careful, controlled hammering which

spreads the force of the blow over the area of the

metal, thus reducing the high spot. When a crown

or curved surface is formed in a metal panel, it

becomes strong in that it resists any change to

its shape. The strength of this crowned surface

can be used to support the surface being hammered

without the use of a dolly. This type of

hammering is called spring hammering, and can

be used to correct high spots on metal panel

surfaces (Figure 13.37). To take advantage of a

great amount of the natural support provided by

the crown of the metal, the force of the hammer

blow is spread over a larger area. Once the metal is

back to its original contour, additional hammering

will cause the surface to sink below its original

contour line, and it may not be possible to raise

it readily. Always start with light blows, and as

the repair nears completion, inspect the work after

Figure 13.37The technique of spring hammering

(Sykes-Pickavant Ltd )

362Repair of Vehicle Bodies

each blow. This will reduce the possibility of sinking

the surface too low. Keep the surface of the

hammer face clean and highly polished. Any marks

on the surface of the hammer will be transferred to

the surface of the metal and create additional work.

Removing low spots

Low spots can be removed in several ways, the two

most common being the use of a pick hammer or

a dolly block. When using the dolly block, start

by holding it so that it can strike the underneath of

the low spot on the panel with one of its rounded

corners. It must be noted that if the operator does

not hit exactly in the centre of the low spot, he will

raise metal in some unwanted place. Accuracy is

therefore essential, and can be achieved by holding

a finger in the low spot and lightly tapping the

underside of the panel with the rounded corner of

the dolly until you feel that it is exactly beneath

your finger, then strike a sharp blow and raise the

metal at this point. After each low spot has been

raised in this manner, these points can be filed

to check that they are level with the surrounding

panel surface.

The second common method of raising low

spots is by pick hammering. Bringing up low spots

with a pick hammer is more difficult than by the

use of a rounded corner of a dolly block. With a

pick hammer more accurate placing of the blow is

required. Likewise greater control over the force of

the blow is necessary. Start using the pick hammer

in a manner similar to the dolly block. Hold the

end of your finger in the low spot. Tap the under

surface of the panel until the pick is directly below

your finger. Then strike a light blow from beneath

the panel, of sufficient strength to form a pimple in

the low spot (Figure 13.38). Care must be taken to

avoid overstretching the metal by using too hard a

blow. These pimples represent stretched metal, but

in being formed also raise the surrounding metal.

When all the low spots have been raised with a

pick hammer in this manner, the pimples can then

be lightly hammered level by direct hammering,

and finished by filing.

13.7 Filing

Filing is one of the most important aspects of

finishing a body panel. It is carried out using an

adjustable file holder, fitted with flexible blades

which can be adjusted concave or convex to

suit most contours on the average vehicle body.

Initially the file was used for smoothing off panels

prior to sanding and locating high and low spots.

With the introduction of body solder and later

metal and plastic fillers, filing took on an even

greater importance in the finishing of repairs on

body panels. Filing indicates any irregularities in

the repaired surface of a panel, and is carried out

as the panel is planished. First of all fasten the

correct file blade to the file holder with the cutting

edges of the teeth facing away from the handle

or operator. Adjust the contour of the file holder so

that it is almost, but not quite, matching the contour

of the surface on which you intend to work.

One hand is used to hold the file handle, while

the other grasps the knob at the opposite end. The

file should be applied with long, straight strokes,

pushing it away from you along the length of

the panel (Figure 13.39). Short, jabbing strokes

should never be used, as these will only scratch

the panel and will not indicate low spots. If the

file digs in, too much pressure is being applied

and hence a need for reduction is essential. At the

end of the first stroke, raise the file and, without

dragging it over the metal, bring it back to the

starting position and make a second stroke. Repeat

this procedure until the area has been covered,

making the file marks parallel to one another. This

Figure 13.38Pick hammering used to remove

low spots

Craft techniques and minor accident damage 363

is termed line filing, and indicates the levelness of

the panel in the direction in which it has been

filed. At this point both the high and low areas

will show up. The high spots can be corrected

by spring hammering and the low spots by direct

hammering, pick hammering, or in some cases by

using the corner of a dolly block. Line filing indicates

curvature in one direction only, and as most

panels are double curved the panel surface must

be cross filed to give an accurate contour check.

Cross filing means a change in the direction of

the file strokes so that the file is moved at an

angle between 45° and 90° over the previous file

strokes, thus checking the accuracy of the curvature

in that direction.

After filing, and prior to refinishing the panel, the

damaged area is sanded using a fine-grit sanding

disc which leaves a smooth, even surface ideally

suited for painting.

13.8 Sanding or disc grinding

Use of grinder

Several general rules govern the use of the disc

grinder. If these are observed they will enable the

operator to become proficient very quickly in the

use of the grinder. The rules are considered good

shop practice and are directed towards the safety

of the operator. In the first instance, if the device

is electrically operated see that it is properly

connected and earthed. Shop floors are usually

of cement; they are generally moist and, therefore,

relatively good conductors of electricity. If the

grinder is not properly earthed it is possible to

receive a fatal electric shock when the machine is

in use. Always wear goggles to protect the eyes

from flying particles of metal and from small

abrasive particles that come loose from the grinding

disc (Figure 13.40). Always replace worn

discs as soon as a tear is noticed; torn discs may

catch in the work and twist the grinder out of

your hands.

Figure 13.39The technique of filing

(Sykes-Pickavant Ltd )

Figure 13.40Ear and eye protection when sanding

(Motor Insurance Repair Research Centre)

Always maintain a balanced position when using

the grinder. This position not only permits perfect

control over the machine at all times, but it

will also produce less fatigue over longer periods.

When operating the grinder, hold it as flat as

possible without permitting the centre connecting

bolt to come in contact with the surface being

ground. Hold the grinder so that only 40–65 mm

of the outer edge of the disc is in contact with the

surface being ground. The grinder must never be

tilted so that only the edge of the disc contacts the

364Repair of Vehicle Bodies

surface. Failure to observe this will cause gouges

or deep scratches in the metal which will be hard

to remove. Move the grinder from left to right,

overlapping the previous stroke with each new

stroke. Make the cutting lines as clean and straight

as possible. Move the grinder in the same manner

whether using it for the removal of paint, rough or

finish grinding. For most grinding operations, finish

grind in the longest direction possible on the

repaired surface.

Sanding discs

The coated abrasive disc is the part of the sander

that does the actual sanding, and selection of the

right grit and coating for each job is important.

There are five different minerals which are commonly

used for manufacturing abrasives. These are

garnet, flint, emery, aluminium oxide and silicon

carbide. Aluminium oxide is the most important

of these. Because of its toughness and durability

it is used in the motor body repair trade, where it

is chiefly used on metal. The abrasive is fixed on

a backing which is either of paper, cloth or a combination

of the two. For dry grinding or sanding,

high-quality hide glues are used for anchoring the

abrasive grains to the backing. For wet sanding,

resins are used as the bonding agent.

Coated abrasives fall into two additional classifications

based on how widely the minerals are

spaced. If the minerals are close together the

abrasive is close coated. When they are widely

spaced it is open coated. In close-coated abrasive

discs, the abrasive is applied in such quantity as to

entirely cover the backing. In open-coated abrasive

discs, the backing is from 50 to 70 per cent covered.

This leaves wider spaces between the abrasive

grains. The open coating provides increased

pliability and good cutting speeds under light

pressures. Open-coated discs are used where the

surface being ground is of such a nature that

closely spaced abrasive materials would rapidly fill

up and become useless. When grinding a body

panel, always use an open-coated disc up to the

time that the area being sanded is completely free

of paint. Then use a close-coated abrasive to grind

the metal to the point where the surface needs no

further correction. The final finish is accomplished

with a fine grit to get the surface smooth enough

for re-finishing by the painting department.

13.9 Hot shrinking

One of the most important skills in the repair of

damaged panels is that of hot shrinking. It is

important because in most cases of collision

causing the damage of body panels, stretching of

the metal takes place. The actual process is

carried out by gathering the stretched metal into

a common centre or area and then by heating

this section. The panel steel is then at its best

condition to be hammered down, thus reducing

the surface area and so making shrinking possible

(Figure 13.41).

Figure 13.41Hot shrinking panel (Sykes-Pickavant )

The oxy-acetylene flame is used as a means of

heating the panel, and care must be taken to reduce

the spread of heat to the surrounding area of the

panel. This can be done by cooling the panel with

water after every shrink. The advisable welding

nozzle size when using for a 1.00 mm panel is a

number 2 nozzle.

Craft techniques and minor accident damage 365