Resto Series Number 2: Sheet Metal Basics Page 2

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Continued from Page 1.

Acquired Characteristics in Old and Damaged Sheet Metal

    The types of damages that can occur to collector car sheet metal are just about unlimited. The most common, by far, are corrosion damage and impact damage. Beyond this, each car that you work on is likely to exhibit some daring innovations in the field of possible sheet metal defects. Stress cracking occurs routinely in some areas of some cars. Wood-framed bodies often exhibit structural shifting that deforms sheet metal, while swelled framing wood can bulge sheet metal in ways that are difficult to resolve. In cars with welded and spot welded attachments, a combination of vibration and corrosion can cause things to break loose and move in ways that produce major messes.

    Yet with all of these possibilities, the damage that I most dread is that done by people armed with minimum knowledge, bad attitudes, heavy hammers and the misconception that they are in the body repair business. When these types and their minions add acetylene torches, plasma arc cutters and pop rivet guns to their basic repertoire of chipped hammers and hardened-screw-tipped slide hammers, they become a definite menace to the welfare of sheet metal everywhere.

    It is sometimes difficult to fathom the degree of imbecility and the resulting destruction that some of these Bondo artists have done to the panels of the poor automobiles that have had the misfortune to come under their hammers. Instead of carefully analyzing the nature of the panel damage that confronts them and repairing it in non-destructive ways, these minor thinkers apply the heaviest hammers or biggest pry bars that they can wield against damaged areas of metal, literally bashing things back toward their right places. In that barbaric process, they produce stretching, further deformation and work hardening that are difficult to correct later.

    When confronted with rust or torn metal, sectioning and butt-welding are usually beyond their limited skill levels, so out come the flanging tools, brazing rods, and pop rivet tools. More damage inevitably follows.

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Howell's Sheetmetal CWI

    These guys buy plastic filler by the 55-gallon drum and the only apparent limit to their use of this stuff seems to be that they never allow the weight of the filler to exceed the weight of the original automobile. Aside from the fact that this kind of work has a life expectancy of between 6 months to 2 years, it always produces severe problems when it has to be reworked by someone who wants to do it right. OK, you’ve been warned. Also, as always, avoid seeing things in stereotypes.

    The two most common forms of sheet metal damage, corrosion and impact, should be dealt with in very specific ways. Corrosion damage must be detected by investigation that employs physically picking and probing, in addition to visual inspection. This may seem brutal, but all kinds of corrosion can be lurking under seemingly sound paint. Certainly, where paint has bubbled and/or blistered, there is good cause to suspect underlying corrosion. A scratch awl is your best guide to its extent. Where body contours appear to be modified, or where panels are 1/8 inch thick, or more, you will often find rust, fiberglass bandages, pop riveted roofing tin and any manner of other mischief underneath the surface.

    Flanged and brazed panel patches are also frequently found under bubbling paint. Sometimes, and this is almost a pleasant surprise, filler will be used to cover dents and other impact damage because the attempted repair involved difficult access to the back of a panel or the individual making the repair lacked the skill and/or commitment to bump the panel to correct its contours. Alas, more often than not in these cases, a slide hammer and hardened screw, body hooks, or welded studs were used to pull dents out crudely, and what lurks under the Bondo is serious corrosion damage, made worse by this kind of attempted repair.

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    The drift of all of this is that the only proper way to repair corrosion damage that perforates sheet metal is to weld in new metal, and the only proper way to deal with impact deformation is to beat it back out in ways that produce the least stretching and buckling of the metal.

    Sometimes, small amounts of filler are necessary. When this is the case, body lead (actually an alloy of tin and lead that is now commonly available in a 30/70 ratio) is really the only way to go in restoration work.

    In addition to the work hardening that occurs in body panels when they are stamped and later subjected to road vibration and flexing forces, there are several other changes in autobody sheet metal that occur when there is impact damage and the attempt to repair it. The most important of these is stretching. When a panel is severely deformed in an accident, it is sometimes stretched. This means that the pressure exerted on it has caused it to become longer or wider, or both. When this happens, it also has become thinner somewhere. Unfortunately, the act of straightening a deformed and stretched panel involves hammering on its ridges and channels, either directly over a dolly block or adjacent to one. This often results in further stretching the metal because metal is made thinner when it is hammered on. Bad repairs often work harden and stretch metal. This can create a difficult combination of defects to address with proper repairs.

    The opposite of stretching is “upsetting,” which sometimes occurs in impact damage but more often is the result of bad repair strategy. This phenomenon involves making an area or areas of the metal in a panel thicker and laterally smaller than it or they were originally. Hammering down a bad buckle directly over a dolly block can produce an upset because the metal may have no lateral place to go. The result is that the upset part of the panel becomes thicker and laterally smaller than it was. This defect must be corrected for the metal to assume its correct original contours. Upsetting can be dealt with in a repair situation and is, in fact, sometimes purposely induced to overcome the effects of stretching. In that case, it is called “shrinking.”

Impact Repair Approaches

    Impact and corrosion damage are sometimes so severe that it is necessary to find replacement panels or to fabricate and section new metal into damaged areas. An example of a small panel fabrication and of section welding are shown and described in the photos and captions that accompany the text of the next chapter. Much of the bodywork that a restorer is likely to encounter involves minor crash damage—dents, scores and the like. It is the complete removal of such damage that can distinguish a very well restored car from one that looks like a near miss.

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Axis Products LeBaron Bonney

    The most important aspect of repairing this kind of damage is to understand the material with which you are working—sheet metal—and to have some general and specific notions of how it got deformed and what kinds of actions will be necessary to return it to its original shape with a minimum of distortion, stretching and upsetting. Remember, a dolly block and hammer used the wrong way can be as destructive as the events that caused the damage that you are trying to repair.

    Proceed in these matters with a very definite plan of attack. Part of that plan should be based on the known sheet metal theory that is described in this book and in the books mentioned at the beginning of this chapter. Another part of your plan will come from your experience, gained from experimentation with scrap panels. The point is, when you swing a body hammer, or decide where to begin to remove a dent, or whether to work “on dolly” or “off dolly,” your knowledge will guide you and your experience will give you an intuitive sense of what the results of a given action will be.

    Prior to the publication of Fairmont Forge’s The Key to Metal Bumping in 1939, such texts that existed in the field of body repair tended to be vague and to stress the black magic aspects of the craft. Sheet metal skills tended to be passed on by oral tradition, which meant that there were some awfully good practitioners and some who were pretty bad. The Key… was a major contribution to the craft because it proposed a simple and very understandable format for sheet metal defect analysis and repair.

    The nugget of the “Fairmont Method” was to logically distinguish between “direct” and “indirect” damage. Direct damage includes areas that have come into direct contact with an impacting object or objects. Indirect damage describes areas that are deformed and locked in by the results of the direct damage, but which were not actually directly impacted.

    Most indirectly damaged areas will spring pretty much back into proper shape if the adjacent areas of direct damage are removed and the forces holding the indirectly damaged areas are thus released. Stamped steel has a memory that promotes this return to original format. Typically, briefcase-sized dents involve mostly indirect damage in terms of the amount of effected surface area. The Fairmont Method prescribes unlocking large expanses in sheet metal that are not deformed beyond their elastic limits by working only on those areas that are. A small “key” unlocks a big puzzle. The revelation of the Fairmont Method is that you don’t have to get a big hammer and pound mindlessly on everything that seems to be pushed in or out in a process that inevitably stretches and work hardens metal unnecessarily and counter-productively.

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    Instead, inspection and analysis will indicate which areas involve direct damage and therefore should be dealt with first. In addition to inspection, the application of logic will yield an understanding of the sequence in which direct and indirect damage occurred. If direct damage is repaired in the reverse order that it occurred, most of the indirect damage will be released as you go along.

    More recent approaches to body damage analysis and repair strategy tend to pay more attention to what is there and less to exactly how it got there. I tend to side with the latter approach but hasten to add that, if you can determine the order of deformation of a particular damaged area, removing the constituents of the damage in the reverse order of their creation is always a good approach. It is not, however, a good idea to waste half a day theorizing about the order of creation of damage, since this is not absolutely necessary information to have in-head before you proceed with corrective measures.

    In any theory of damage analysis and repair strategy, the damage itself is reduced to one or a combination of three possible constituent parts. These are V-channels, ridges and buckles (also called “rolled buckles”). These three categories, and their almost infinite combinations, cover the field. Ridges, as the name implies, are areas of raised metal, which stand out in a linear formation. V-channels are depressed areas formed into lines, the opposite of ridges. Buckles are areas that are forced and locked into the metal by the waveform created in the metal by the original impact.

    Unlike ridges and V-channels, which are either results of direct damage or fairly gentle extensions from it, buckles are formed by the collapse of the metal when it is under pressure and literally has no alternative other than to collapse. Buckles often involve substantial upsetting, which is not the case with ridges and V-channels.

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    When you recognize and understand the genesis of these three components of damage, you will be in a position to execute an effective strategy for their removal. In large part, your actions should unlock what are usually large areas of indirect damage.

    In a sense, the test of a good strategy is how little hammer and dolly work is necessary to remove damage. The analysis method works because breaking damage into components, and attacking those components logically, represents an efficient attack on the causes of the problem. The alternative, to mindlessly attack the symptoms of damage, ends up as the “bigger hammer” approach and usually fails to recognize even such obvious components of damage as bent substructure. It substitutes damaging counter-force for intellect and skill. For that reason, it usually fails.

Now the metal is gripped between the pliers’ jaws and compressed slightly.
The metal strip is now gripped in the pliers as close to the bend as possible, and an attempt is made to bend it back straight by hand.

The area of the first bend refused to bend back straight, and the metal on either side of it has yielded to the reverse bending pressure first. This is because the metal in the original bend was work hardened and provided more resistance to bending than the unbent metal on either side of it. Without some further intervention, this is as straight as the author’s wife can get the steel strip with her hands and a pair of sheet metal pliers. This is a visibly dramatic demonstration of the work hardening phenomenon. It also is very similar to what happens when you attempt to hammer a crease out of a fender by hammering directly on the crease.
To really straighten this strip, and to overcome the work hardening in its bend, would take mechanical force, as is shown here. This will tend to stretch the metal, unless it is done very gently. Keep these characteristics of sheet metal in mind when you go to straighten out a ridge, V-channel or buckle in a mild steel panel.