Automotive Bodywork and Rust Repair

by Matt Joseph

  that are mostly judged on their

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  AUTOMOTIVE BODY WOR K AN D R UST R E PAI R

  W HAT YOU S HOU LD K NOW B E FOR E YOU START

  content than iron, the even dispersal

  of what carbon it does have makes it

  strong and somewhat plastic, or

  deformable, unlike various irons.

  Mild sheet steel, the stuff of auto-

  bodies, is roughly .25-percent car-

  bon. Above that concentration of

  carbon, steels begin to fit into the

  medium steel classification. Between

  .6-percent and 1-percent carbon,

  steels are considered hard or high-

  carbon. Ultra hard steels, like tool

  steels, may contain between 1-per-

  cent and 2-percent carbon.

  How panels are supported makes a tremendous difference in how you

  The softness of panel steel allows

  approach their repair. This 2008 Mitsubishi Galant’s upper fender attachments

  it to undergo the highly organized

  are very unusual. Short strut pieces attach the fender tops to the car’s inner

  brutality of stamping it into complex

  fenders. Anyone who repairs these fenders has to take this into account.

  three-dimensional shapes like doors,

  hoods, roofs, and fenders. Using heat

  visual merits. At times, and in some

  truly amazing material. It is a com-

  and enormous pressure, automotive

  situations, a good practitioner uses

  plex alloy of iron, carbon, and other

  body steel is stamped into final sheet

  characteristics of panel configuration

  elements. It has been heat treated in

  format. While it is primarily an alloy

  to slightly trick the eye. (There will

  its manufacture to disperse the car-

  of iron and carbon, several other ele-

  be more on this topic, later.)

  bon evenly into the steel’s granular

  ments—which, in some cases, have

  Along with crown, how a panel

  structure. While steel has less carbon

  names that are hard to remember and

  is supported and attached to a vehi-

  cle is critical in understanding how it

  performs under impact, and how

  best to remove impact damage from

  it. Many panels have strengthening

  structures welded or bolted under

  them. Panels that are attached to

  vehicles by welding them to sub-

  structure perform differently from

  those that are bolted to substructure.

  Unless you deal with them, bent or

  damaged

  substructure

  reinforce-

  ments and fastening points that

  impart strength to panels, cause pan-

  els to resist restoration to their origi-

  nal formats. Always consider this

  factor when you plan panel repair or

  restoration work.

  Autobody Steel

  Throughout most of automotive history, all panels were stamped out in

  presses, like the ones shown here in a General Motors stamping room in the

  The steel sheet stock that is

  mid 1970s. More recently, some very large stampings are rolled into panels by

  formed into automobile panels is a

  dies that move in two dimensions. (Photo supplied by General Motors Corp.) AUTOMOTIVE BODY WOR K AN D R UST R E PAI R

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  CHAPTE R 1

  difficult to pronounce—are routinely

  and repair: plasticity and elasticity.

  added to it to give it the special char-

  Plasticity is the ability of metal to

  acteristics that are needed to form it

  deform without fracturing. The point

  into automotive panels.

  of fracture is called the “yield” point.

  New car panels are presently

  Automotive panels are stamped at

  in the range of 22-gauge to 23-gauge;

  the factory from flat stock into com-

  that is, .0299 and .0269 inch.

  plex, three-dimensional shapes. The

  Note that as the gauge number

  fact that this can be done is proof of

  increases, the thickness of steel sheet

  their plasticity. When a body repair

  stock gets thinner. The way that this

  Thin panels are hard, presenting

  technician works on them with ham-

  works involves an arcane formula

  several problems in repair. It is easy

  mers, dollies, and other tools, they

  that takes into account the weight of

  to cut through, when welding them.

  are again deformed, courtesy of their

  a cubic foot of the material involved.

  Their hardness and thinness make

  plasticity.

  To make things thoroughly confus-

  them difficult to file because files

  Plasticity under tension is called

  ing, basing gauge on weight means skitter over them, rather than cut in.

  ductility, and produces stretching

  that the same gauge number applied

  Worse, very little metal can be

  when it occurs. Think of the bumper

  to different metals gives different

  removed before they become

  over-rider on a truck smashing into

  thicknesses. For example, while dangerously thin.

  the door of your vehicle. It deforms

  22-gauge sheet steel is .0299 inch

  it—plasticity—and it probably will

  thick, 22-gauge galvanized steel is .031

  els. In most cases, the thinner that

  put the metal under tension and

  inch thick, 22-gauge aluminum sheet

  body metal is the more problems it

  stretch it—ductility. When plasticity

  stock is .025 inch thick, and 22-gauge

  tends to present in repair. That is

  occurs

  under

  compression,

  as

  stainless steel is .031 inch thick.

  because the thinner body metal is,

  opposed to tension, it is called mal-

  The important things to remem-

  the more difficult it is to form and to

  leability, and produces the opposite

  ber are that as gauge numbers

  weld. The alloys used in thinner

  of stretching by compacting or

  increase, thickness decreases, and

  panel sections tend to be harder than

  “upsetting.” In upsetting, metal is

  that the same gauge numbers for dif-

  the older, thicker panels, because

  piled into itself.

  ferent metals may translate into

  they contain more carbon. That

  Let’s go back to that unfortunate

  slightly different thicknesses.

  makes them more difficult to deform

  damage to your vehicle’s door that

  Finally, there is a misconception

  with body tools, without taking

  occurred when a truck hit it. After

  that gauge designations involve the

  them beyond their yield points (frac-

  the accident, a technician removed

  number of sheets of a particular

  turing them). Their hardness also

  the inner panel from the door. Then,

  gauge that can be fit into 1 inch.

  makes them very difficult to surface

 
the technician began to fix the dam-

  This, simply, is not true. Common

  file for the purpose of leveling them.

  age by hammering the ridge near the

  gauge numbers for automotive

  Welding thinner metal is always

  center of the dent down and out

  outer-body steels are:

  more challenging, due to the ten-

  against a dolly, centered under it on

  dency of thinner sections to melt

  the outside of the door. If the techni-

  • 18-gauge .0478 inch

  and “drop out” at welding tempera-

  cian had read this book, he or she

  • 19-gauge .0418 inch

  tures. That outcome also can be very

  would probably have had a better

  • 20-gauge .0359 inch

  hard on a metal worker’s shoes.

  first move. The accident probably

  • 21-gauge .0329 inch

  stretched the metal in the door’s skin

  • 22-gauge .0299 inch

  Plasticity and Elasticity

  because it was deformed while being

  • 23-gauge .0269 inch

  held rigidly at both ends by the

  • 24-gauge .0239 inch

  When I speak of the hardness of

  door’s substructure. The attempt to

  metal, I am generally describing sev-

  hammer it out put the area near the

  Thickness is important because,

  eral significant characteristics, two of

  hammering

  under

  compression

  in part, it determines how difficult it

  which are particularly important to

  because the dolly was supporting the

  will be to repair damaged body pan-

  anyone working in panel fabrication

  undeformed metal on either side of

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  AUTOMOTIVE BODY WOR K AN D R UST R E PAI R

  W HAT YOU S HOU LD K NOW B E FOR E YOU START

  SUV, and had the queasy sensation of

  feeling your hand deform the hatch

  metal where you were pushing

  against it. But then, as you released

  the panel, you felt the metal under

  your hand return to its rightful

  shape. You can thank elasticity for

  that good outcome. If the metal did-

  n’t spring back, it was because you

  Upsetting can be useful. Here, it is

  exceeded its elastic limit.

  Good news! This dent looks worse

  used to shrink a stretched area. The

  Elasticity is critical because dam-

  than it is. Most of the displaced metal

  metal is heated until it bulges, and

  aged panels usually contain a small

  is being held out of place by the ridge

  then hammered down. The hot metal

  minority of surface area that has

  in its middle. Once that ridge is

  piles into itself because it is bounded

  been pushed, or deformed, beyond

  unlocked, most of the damaged area

  by unyielding cold metal. The

  its elastic limit. Most of what may

  will spring back into its proper place,

  resulting upset makes the heated

  look like damaged metal—because it

  on its own.

  area thicker and laterally smaller.

  is out of position—has not been

  deformed beyond this limit, and will

  under the cover of a pile of books or a

  the ridge. The result of hammering

  return to its pre-accident shape when

  knapsack, so that your teacher would

  down on the obvious ridge, with a

  you release the small areas of badly

  not see you performing this metallur-

  dolly under it, was to compress the

  deformed metal that are holding it

  gical experiment—was that before

  metal there latterly, or to upset it.

  out of place in the damage. I don’t

  any of the three bends in the paper

  This is a critically important dis-

  want to sound excessively rosy about

  clip could be straightened, the metal

  tinction in autobody work. When

  these matters but, to the untrained

  stopped moving in the bends and

  you stretch metal you are effectively

  eye, panel damage almost always

  bent on either side of them, leaving

  exchanging some of its thickness for

  looks worse than it is.

  shapes like saddles between two

  increased lateral dimension. When

  opposite-facing humps, in kind of a

  you upset metal, you are exchanging

  Work Hardening:

  camelback configuration. The saddles

  some of its lateral dimension for

  The Metal Remembers

  were what was left of the original

  increased thickness. At various

  bends. The humps were new bends,

  points in working with body metal,

  The great elephant hiding dis-

  in the opposite direction, that

  you need to create upsets, and even

  creetly in this sheetmetal living

  occurred when the metal in the origi-

  stretches, on purpose. At other times,

  room is called work hardening. This

  nal bends stiffened as you bent it, and

  you will need to avoid these dimen-

  is the tendency of metals, like mild

  approached its elastic limit. Then, the

  sional transformations, or have to

  sheet steel, to become progressively

  opposite-facing humps were made as

  correct them. It is critical that you

  harder as they are deformed beyond

  you continued to apply pressure.

  understand exactly what stretches

  their elastic limits.

  That poor paper clip began its

  and upsets are, and why and how

  Doubtless you have already per-

  life as a straight piece of wire. Form-

  they occur. Later, I will discuss how

  formed experiments involving this

  ing it into a paper clip work hard-

  to purposely create them, and what

  factor, although you may not realize

  ened the metal in its bends. When

  situations call for creating them.

  it. If you, like most people, ever tried

  you tried to straighten it, you made

  Elasticity in metal is its ability to

  to straighten out a paper clip with

  some progress, but work hardening

  flex to a limit—its elastic limit—and

  your fingers, you encountered work

  made complete straightening impos-

  still return to its original shape, on

  hardening. What you discovered was

  sible, so the metal bent on either side

  its own. Some call this characteristic

  that it is all but impossible to get the

  of the work-hardened area. This is

  memory, or spring back. You might

  three bends out of a paper clip with

  not trivial. Work hardening is terrifi-

  have encountered this when you

  your bare hands. What happened

  cally important in body work. You

  slammed the hatch on a minivan or

  when you tried to do this—probably

  must learn to identify it, predict it,

  AUTOMOTIVE BODY WOR K AN D R
UST R E PAI R

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  CHAPTE R 1

  An Example of Work Hardening

  Here is a simple but dramatic example with his fingers. But the bend has work first work-hardened bend stubbornly of the work-hardening effect.

  hardened and the metal wants to bend

  refuses to budge.

  Herb clasps a strip of 22-gauge mild

  everywhere else, in the non-work-

  Finally, Herb is able to hammer the

  steel in a pair of sheetmetal pliers and bends

  hardened metal, and not in the first bend

  original bend and the side bends flat on

  its middle to as close to a right angle as the

  that he made. Frustrated, Herb tries to

  an anvil. However, evidence of all three

  jaws of the pliers al ow. Then, he closes the

  straighten the bend by holding the metal

  bends remains visible on the flattened

  bend as far as he can in the pliers’ jaws.

  in the pliers and forcing it, but that doesn’t

  piece.

  After removing the strip from the pli-

  work. Then, he tries to straighten it with his

  This sequence is a testimonial to the

  ers, Herb attempts to straighten the bend

  hands against a wood table top, but the

  persistence of work-hardened metal.

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  AUTOMOTIVE BODY WOR K AN D R UST R E PAI R

  W HAT YOU S HOU LD K NOW B E FOR E YOU START

  Annealing Effects

  One way to mitigate work-hardening

  effects is to anneal metal. In this

  process, metal is heated to its critical

  temperature, roughly 1,600 degrees F in

  the case of mild sheet steel, and allowed

  to cool slowly in air. The effect is to relieve

  the metal’s stiffness and reverse the work-

  hardening effect.

  In this demonstration, a strip of sheet-

  metal is bent as close to a right angle as it

  is possible to do with bare hands. Then,

  unlike the demonstration of work harden-

  ing, it is heated with an oxy-acetylene torch

  to roughly 1,600 degrees F and allowed

  to cool.

  Now, it is easy to straighten the bend

  with bare hands. The two strips were bent

  almost identical y. Both were straightened by

  hand, one with annealing and the other with-

  out it. It’s pretty easy to tel which is which.

  and deal with it, because it tends to

  in your frustration, when you

  Heat also has the ability to

  be a factor in almost all of your colli-

  couldn’t straighten that paper clip,

  rearrange those grains for important

  sion damage and fabrication efforts.

  you bent it back and forth until it