Making It Straight Again

Jan. 1, 2020
Once common, mild steel is a rare material today. The quest for lightweight structures and fuel efficiency has dramatically increased the use of stronger, lighter components.

This is not your father's sheet metal. The use of high-strength steel requires less aggressive repair techniques.

Once common, mild steel is a rare material today. The quest for lightweight structures and fuel efficiency has dramatically increased the use of stronger, lighter components. Door intrusion beams are typically ultra high-strength steel (UHSS) and are not repairable. Structural components such as frame rails and strut towers are typically high-strength steel (HSS) and limited repair techniques apply. But outside panels such as quarter panels, hoods, and deck lids are also more frequently HSS. Repairing these panels requires an understanding of the metal and the techniques needed to repair it.

Sheet metal is the common term used when referring to exterior panels, because that is what was used to make almost all panels on vehicles until relatively recently. Mild steel was easy to repair compared to today's higher strength sheet metal parts. Competing for use in exterior panels is aluminum, which requires very different techniques. This article will deal only with steel.

Shaping metal with hand tools is one of the basic skills practiced by collision repair technicians. Flat steel was once hammered by hand into fenders and doors to produce the first car bodies. Some very limited production vehicles are still at least partially hand made. Custom car shops use tools such as leather bags filled with lead shot and large, round headed hammers, stretching and shrinking tools, rollers, sheet metal breaks and English Wheels. More basic tools and skills are used for common dent repairs.

The New and Improved Steel: A Material Science Lesson

Mild steel is very malleable, meaning it is easily formed and worked. Mild steel panels were as much as twice as thick as current production models. Hammering, grinding, filing, and heating were all very effective techniques for repairing damaged panels. As performance goals shifted, high-strength steel replaced mild steel in many panels. Making steel stronger is done in two ways. Changing the chemical composition can alter the physical properties of steel. Using heat to treat the metal also changes the steel. As with most things in life, there are tradeoffs involved. Mild steel is very easy to work with. Heating and cooling it does not change its strength. Hammering or bending it will slowly work harden it, but heating and slowly cooling it will anneal it back to mild steel softness. Because the mild steel panels are relatively thick, a little metal removed with a file or grinder is a small compromise.

However, HSS can be altered permanently with any of these actions. Steel that has been modified to be stronger also becomes more brittle. This is most clear in UHSS, used in door intrusion beams and bumper reinforcements. This steel is extremely strong, with a tensile strength of 170,000 psi or more, as compared to 30,000 psi for mild steel. But if this steel is bent, it will usually crack. It has very little ability to be formed once the manufacturing process is complete. This is why vehicle manufacturers prohibit repairs of UHSS other than minor bends at mounting points. Heating HSS or UHSS will remove the heat treatment that was used to produce the desired mechanical properties in the first place. When working with exterior, non-structural panels, few manufacturers prohibit heating, but it may make working with the metal more difficult.

Roughing the Dents Out

There are several ways to repair a dent. Usually the metal has been deformed enough to require several steps. The goal is to move the metal back to its original shape, or as close as possible, without causing more damage. The simplest method is gaining access to the back side of a panel and hammering it out. The common hammer and dolly are the preferred tools for this job. Just as a blacksmith uses an anvil and hammer, the body man has a dolly as a portable anvil and a hammer is used to pound the metal back into shape.

Dollies come in several shapes and should always be paired with the proper hammer. A flat-headed hammer is used with flat or curved surface dollies, while a round-headed hammer should usually be used with flat dollies. The goal is to engage large areas of sheet metal with each blow. This avoids stretching the metal needlessly. This skill looks easy to the novice. Developing a feel for the metal takes year of practice. Experienced craftsmen tend to use lightweight hammers and an almost delicate touch with the metal.

Alternative Dent Pulling Tools

It used to be that holes were drilled in panels and dents were pulled out with hooks or a screw on a slide hammer. This provided a very good path for moisture and rust. The stud welder was a welcome addition to the tool set. The tool is a small spot welder. It is used to attach copper coated steel "nails" to panels. Then a slide hammer or T-handle is used to pull the dent out. The stud is then twisted, cut or ground off.

A recent addition is hot glue dent pulling. Hot-melt glue is used to attach a small tool, which is then used to pull the dent. Heat or solvent is used to remove the adhesive. This was introduced as a technique for roughing out small dents and has migrated into paintless dent repair (PDR).


PDR is a simple technique, which uses a variety of tools to gain access to the backside of a damaged panel and push the dent out without damaging the paint. For small, symmetrical dents such as hail, this process can work very well. There are two major concerns. One is that the corrosion protection either should not be damaged or should be restored after repairs. Second is that the paint really is not damaged. Small fractures or excessive sanding and buffing can compromise this type of repair. Plastic-tipped tools have been introduced as one way to avoid scratching the e-coat when performing PDR.

Shrinking Metal

A common process with mild steel is using a small flame and a wet rag to shrink the panel. When steel is heated to "cherry red" it expands. To shrink a spot in a panel, a spiral pattern is traced with the tip of the oxy-acetylene torch until a small red hump is produced. This hump is pounded flat with a hammer and quickly quenched with cold water. While the mild steel is red, it is very easily formed. Flattening the high spot causes the structure to change. When cooled it draws the surrounding metal in, shrinking the panel. With HSS, this procedure may not achieve the desired result. Because the metal is thinner, it is difficult to not heat a large area. The stiffness of much of the panel may be changed, or a much larger area may be shrunk than desired. Electric tools have been developed to shrink modern panels more easily. A rounded head on a stud welder heats the metal and then quickly absorbs the heat from the panel, shrinking a small spot.

Spreading Mud

Whatever process is used, the final step will usually be plastic filler. Polyester resin combined with various fillers such as talc, clay and hollow micro-spheres is activated with an oxidizing agent to produce a sandable filler. The performance of plastic body filler is outstanding when compared to the brittle stuff of years past that used to fall off panels if they were hit in another accident.

Many OEM recommendations call for bare metal to be coated with epoxy primer before applying body filler. Many body fillers have additives intended to make them compatible with use over galvanized metal. Some of the final coats are intended for use over stable, sanded paint. If you are removing the paint from a panel, the least aggressive method should be used. While 24 and 36 grit grinding discs are good at removing metal after welding, care should be taken not to remove too much metal which could weaken the panel unnecessarily. For removing paint in preparation for spreading body filler, 80-grit sand paper or plastic abrasive discs are a better choice. E-coat and galvanize coatings should be left intact when possible.

Mixing and spreading body filler is another simple task that is mastered by few. When mixing plastic, a folding or stirring action is desired to avoid adding air into the mixture. Tiny air bubbles in plastic become pinholes when sanded open. The first coat should be placed with relatively high pressure to achieve good mechanical contact with the surface and avoid trapping any air. This is much like the scratch coat used when applying plaster. Successive coats may be applied with less pressure to produce the desired shape. Once hardened, the plastic is sanded to shape. Most primers require a surface with no sanding scratches coarser than 180 grit.

The process of repairing collision damage on exterior panels requires some understanding of the materials under repair, an assortment of tools, and of course, the talents of a skilled technician.

About the Author

Charles Wilhite

Charles Wilhite, owner of Gamut Services, provides training and consulting to the automotive colli-sion industry. He is an active I-CAR instructor, I-CAR Advantage contributor and an ASE-certified Master Collision Repair/Refinish Technician with more than 25 years in the trade. He also teaches composite construction and repair in the aviation industry.

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