Automotive Bodywork and Rust Repair

by Matt Joseph

  lurk on metal, under coatings.

  great work. Of course, small mis-

  If you used no filler, or used it

  As I said, if your basic approach

  steps can ruin a job, but it usually

  very sparingly and in very minor

  was right, and you supported that by

  takes more than a single slipup to

  thicknesses, you have a definite ben-

  getting all, or an overwhelming

  botch everything.

  efit. If you used lead filler, and were

  majority, of the pertinent tasks and

  If you bumped or fabricated metal

  careful to neutralize all traces of flux

  details of your work right, it should

  to very good approximations of final

  and tallow residues as you went

  rank at or near the highest possible

  shapes, leaving little for filler to fill,

  along, that is a big plus. If you used

  grade. It should look good and last

  this helps. If you attended to shrinks

  plastic filler and were careful to thor-

  long, and that is a substantial accom-

  and stretches in the metal, as you

  oughly mix its components, individ-

  plishment, one in which you should

  formed it into correct shapes, you are

  ually and with each other in correct

  take considerable pride.

  a long way toward making its final

  proportions, that removes two more

  format stable. If you avoided putting

  possibilities of failure at a later date.

  The Danger from Behind

  stress into the metal by forcing it

  Whichever kind of filler you

  harshly into position, and then weld-

  used, if you were meticulously care-

  There is one more critical detail

  ing it there, this helps some more. It

  ful to sand out all file marks, and

  that accompanies the completion of

  is okay to slightly fine tune an edge

  then to use abrasives with escalating

  metal work, before you paint it your-

  with a small screwdriver, persuading

  grit numbers to erase all deep

  self or turn it over to the paint guys.

  it into perfect position for welding.

  scratches left by previous and coarser

  It is the consideration of protecting

  However, if you made things line up

  grits, until you had produced a sur-

  the back side of your work from

  with pry bars and 2-pound hammers,

  face free of visible individual

  attack by moisture and the corrosion

  that will invite big problems, later.

  scratches, you have won the battle

  that inevitably follows prolonged

  If your metal finishing achieved

  against sand-scratch swelling in the

  contact with moisture. If your work

  perfect contours, as revealed by care-

  finish that will cover your work. If

  fails at some future date, due to cor-

  ful visual and tactile inspections, the

  your last sanding was with an abra-

  rosion, it may be difficult to deter-

  odds in favor of it turning out

  sive that left both metal and filler

  mine the source(s) of that failure.

  between very well and perfectly are

  surfaces with good tooth for primer

  Perforation rust can originate on

  increased. If you made your filler

  adhesion, you have headed off

  either side of a panel. When you do

  application over metal that was com-

  another potential issue that can

  great metal work, protecting the

  pletely cleansed of all traces of rust

  plague metal work. If you cleaned

  back sides of your panels makes great

  and other contamination, you have

  your finished surface with solvent,

  sense. It won’t happen unless you do

  another considerable advantage.

  and then blew off all loose debris

  it or pay to have it done.

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

  111

  CHAPTE R 11

  ings hopelessly contaminated with

  what was supposed to be kept out

  and, due to their seal designs, all but

  impossible to relubricate? You have

  to be vigilant to avoid creating your

  own versions of these counterpro-

  ductive situations. Providing for fac-

  tors like drainage, venting, and

  cleaning access usually helps.

  Consider the sealing systems that

  wipe against and seal car door win-

  dows. They are designed to keep water

  from seeping into doors, as windows

  are lowered, and rusting them out at

  vulnerable seams. Those vehicle door-

  The bolting flange at the bottom of this front fender is problematical because

  window seals keep most of the water

  it traps moisture and dirt. Aside from keeping it clean, the best in-service

  that could enter that way from leaking

  protective measure is to coat it with a strongly bonded, dense paint, followed

  into doors. Unfortunately, they also

  by a good rust-proofing agent.

  help to seal in the moisture that does

  get past them and that arrives from

  Your first chance to deal with

  quences, a variant of Murphy’s Law.

  other sources. Then, when the sun

  potential corrosion is when you

  My favorite formulation of Mr. Mur-

  beats down on the outsides of these

  design a sheetmetal structure, or work

  phy’s dictum is the statement that

  doors, with the potential to heat their

  on one that has obvious problems in

  “nature always sides with the hidden

  cavities, evaporate the moisture there,

  this area. In the main, areas that trap

  defect.” When dealing with corrosion

  and drive it out, those same wiper seals

  water or, worse, water and dirt, and

  prevention in sheetmetal structures,

  help to prevent this desirable outcome

  hold them against metal are places

  you have to identify all hidden defects

  from occurring. The holes and vents in

  where corrosion is likely to begin and

  and take countermeasures to over-

  door bottoms that are designed to let

  increase rapidly. Dirt tends to absorb

  come them. Then, you have to make

  water out often foul with dirt and

  water, and hold moisture against

  sure that none of your countermea-

  debris, blocking its exit. Over the years,

  metal, long after just water would

  sures created new hidden defects

  a few automotive body cavities have

  have left the scene due to evapora-

  along the way. If this happened, you

  been fitted with vents designed to

  tion, passing air, or by the momen-

  have to take countermeasures against

  work on the Bernoulli principle—to

  tum generated by vehicle movement. the flaws in your countermeasures.

  use passing air to create a low-pressure

  One major corrective step that

  For example, tak
e the matter of

  situation to extract water from their

  you can take is to avoid designing

  sealing structures from the intrusion

  insides. This works until some small

  structures with moisture traps that

  of moisture. Consider something like

  bit of debris changes their configura-

  hold water, and small openings that

  a headlight module system in failure,

  tion, and then becomes useless.

  can use capillary action to draw

  with fog coating the inside of its

  In many situations, keeping

  moisture into narrow spaces. If you

  lens. It was designed to keep mois-

  moisture away from metal is a very

  can imagine some of its features

  ture out but the design not only let

  tricky proposition. I don’t care if

  acquiring and/or trapping water

  moisture get in, it actually trapped it

  they say that something difficult is

  when you look at a structure, you

  there by preventing its exit. That is

  like making water run uphill. Take

  should try to prevent this from hap-

  why it remains there. Another exam-

  my word for it, water will run up hill

  pening or to figure out a reliable way

  ple is the seals on some bearings.

  enthusiastically, when it senses that

  to cause them to drain.

  They were designed to keep lubricant

  it can find something to rust by

  At this point, you can very easily

  in, and dirt and moisture out. How

  going there. Okay—I haven’t actu-

  run into the law of unintended conse-

  many times have you seen such bear-

  ally seen this happen.

  112

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

  B E FOR E YOU PAI NT

  Failed anti-corrosion designs and

  priate for finishing vehicle topsides, it

  features are not hard to find. They

  is ideal for protecting their undersides.

  are all around us. Just check out any

  It clings to properly prepared metal

  junkyard.

  tenaciously, blocks the transit of mois-

  Still, you shouldn’t be over-

  ture, and remains resilient for decades

  whelmed by the odds against taking

  after it is applied. This means that it is

  perfectly effective action to prevent

  unlikely to crack under the assault of

  rust from mortally attacking your

  flying gravel and other things kicked

  sheetmetal work someday. Just try to

  up by vehicle wheels.

  put that day off for a century or so.

  Seam sealer is a terrific way to pro-

  After you have completely sealed

  You have a big, last chance to make

  tect vulnerable joints, such as this

  the back side of your metal work, go

  your final moves just before your pro-

  door skin joint, if it is applied over

  for some extra insurance. Remember,

  ject is painted. Later, you may have a

  scrupulously clean metal and in a

  anyone who wears both a belt and

  few more chances to inspect, detect,

  reasonable thickness. It is also

  suspenders is unlikely ever to suffer

  and correct problems, but remember

  possible to abuse seam sealer

  the embarrassment of lower wardrobe

  that corrosion looks for ways—24/7

  applications in ways that cause them

  failure. The extra insurance is to coat

  and holidays—to destroy your work.

  to do as much harm as good.

  the painted underside of your work

  with a flexible and resilient corrosion-

  What You Can Do

  The third line of defense is seal-

  protection agent. This could be a hot-

  The good news is that there are

  ing and coating the unseen sides of

  sprayed, paraffin-based rustproofer

  things that you can do. First and

  your work. It starts with removing all

  that contains anti-corrosion addi-

  foremost, you can try to keep water

  traces of paint, rust, grease, and oil

  tives, or a rubberized undercoating.

  from seeping in. This means design-

  from the back sides of your metal

  Finally, you should check these

  ing seams and joints that are, and

  work. After that, apply a good seam

  measures, after their applications

  remain, tightly sealed against mois-

  sealer, particularly to the back sides

  have settled, to make sure that there

  ture. But that is not enough because

  of lap and offset lap joints. Body

  are no gaps or other flaws in them.

  moisture is pernicious; it can enter

  caulk should be used in areas that are

  For example, be sure that you applied

  areas in airborne form, and condense

  too wide open for seam sealers to

  the undercoating consistently. And

  into liquid. Even in airborne form, it

  fully close them. Back side metal

  can start and promote corrosion,

  should be coated with a waterproof

  without ever becoming a liquid.

  and very dense (in the molecular

  Keeping water out is a noble aim,

  sense) coating like moisture-cure ure-

  and should be pursued. But do so with

  thane. Applied to steel, moisture-

  the certain knowledge that your suc-

  cure urethane aggressively draws

  cess with it will be partial at best. With

  water molecules from both of its sur-

  that in mind, there is a second line of

  faces, the one exposed to the envi-

  defense that you can pursue. If the

  ronment and the one facing metal.

  structure that you wish to protect is

  That means that it removes all mois-

  one that you designed, you can elimi-

  ture from what is under it—a consid-

  nate obvious water traps like shelves

  erable advantage in fighting rust.

  Hot paraffin-based rust-poofing

  and other enclosures. You may

  The molecular structure of a mois-

  material is great for protecting most

  include drains for areas that might

  ture-cure urethane coating is so dense

  undercarriage surfaces, where the

  otherwise possibly trap water. You can

  that even the relatively small, freneti-

  impact of gravel and stones is not a

  vent potentially vulnerable areas, so

  cally active H2O molecule has great

  problem. Where it is, a good

  that moisture has a chance to escape,

  difficulty penetrating it. While mois-

  rubberized undercoating works best.

  or is extracted from them. You can

  ture-cure urethane’s surface character-

  The coatings shown here are applied

  make your vents and drains large

  istics, and its lack of robust resistance

  with the air sputter gun, shown in the

  enough to not plug up with debris.

  to ultraviolet light, make it inappro-

  can on t
he left.

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

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  CHAPTE R 11

  be sure that it didn’t flow downward,

  Accord in for rustproofing that day.

  that Honda’s designers and engineers

  potentially creating gaps, and possi-

  He walked me over to it, drilled an

  had not. Within a few years, almost

  bly clogging or blocking critical

  access hole in its right front fender

  every early Accord that one saw in

  drains and/or vents.

  door-jamb-facing

  surface,

  and

  northern climates was either rusting

  With all of those measures, you

  inserted his undercoating wand

  about its fender tops, or sporting

  may still fail to provide your work

  through the hole. He told me to

  replaced fenders. Honda even offered

  with perfect, or even adequate, protec-

  watch the metal on the top of the

  replacement fenders for little or

  tion against rust. This is true for both

  fender, near the cowl and hood, as he

  nothing, if the customer would pay

  custom work and for modifications

  shot undercoating at it. Then, he trig-

  for installation and painting. This

  that you may make to improve the

  gered his spray wand and withdrew it

  offer was kept open for a long time

  corrosion resistance in existing vehicle

  through the drilled hole, spraying

  after the original warranties on these

  designs. Sadly, in the end, rust always

  high-pressure, hot paraffin under-

  cars had expired.

  triumphs. The real issue is what we

  coating from its nozzle at the fender

  I tell this story to make several

  can do to deny it that victory for the

  top’s underside. As I watched, I saw

  points. Honda had very good body

  longest possible time. If we gain some

  the metal in the fender’s top deflect

  engineering in that period, but failed

  valuable time against the rust enemy,

  slightly, from the pressure of the

  to foresee this problem with the

  that alone is worth celebrating.

  undercoating hitting its other side.

  Accord. Ignorance of the North Amer-

  What I have written on this topic

  “Wow,” was all that I could say.

  ican climate and of salt use here prob-

  indicates how tricky rust prevention

  My friend agreed, and noted that he

  ably contributed to creating this

  problems can be, and how carefully

  had never seen anything like it.

  design flaw. Besides, these kinds of

  you have to develop and deploy your