The finer points of metal finishing

Jan. 1, 2014
Consider this your refresher course on the art of dent repair.
Figure 1

Dent repair is a highly skilled task that, when done correctly, may exceed the cost of replacing a part. Unfortunately, this means that precise and efficient metal repair may not be the most economical repair procedure. Metal finishing — restoring damaged metal to its pre-accident condition with little or no filler — has become a less-practiced art, and some would argue that it may soon become a lost art.

Figure 2

Some say that the thinner, stronger metals do not metal finish as easily as the older, softer and thicker ones do… and they would be correct. Try to bump a high-strength steel compared to, say, an older 1960s fender. The way the older metal moves is much different than the newer, but it does not mean that the repair can’t be done on high-strength surfaces. In fact, in many areas outside of North America, repair is the method preferred over replacement.

One critical decision when restoring a vehicle to its pre-accident condition is determining what method best fits the cycle time and the needs of the customer, which may mean that replacement is the best choice. Metal finishing, though, is a valuable skill that new and old technicians alike should be capable of performing efficiently. Keeping a few tips in mind will help you to be confident and successful when you choose metal finishing.

Figure  3

Assess the damage

If both sides of the damage are accessible, methods such as hammer and dolly repair can be used (Fig 1), or the use of a pick bar can help raise the low spots (Fig 2). If both sides cannot be accessed, stud welds (Fig 3), wiggle wire, and progressive pulling are other methods that can be used. In fact, there are numerous single-side repair methods such as glue-on pulling, suction cups, and heat shrinking, just to name a few. What a technician must do is evaluate the repair to come up with the plan that will be the most efficient and profitable.

Make a repair plan

Like most repair processes, metal finishing should start with a plan. First, inspect the damaged area. Determine what part is direct damage — the damage caused by

Figure 4

the impact — and which part of the damage is indirect damage. Indirect damage is caused not by the object that struck the vehicle, but by the deforming of the panel from the impact.

In Fig. 4, the impact in front of the vehicle shortened the vehicle, causing the indirect (sometimes called secondary) damage in the roof.

Figure 5

Indirect damage is often larger in area than the direct damage, and if the repair is started in the wrong area, more damage can be caused than initially existed. As an example, in the damage in Fig. 4, if the repair is started in the roof area, it would be nearly impossible to repair until the direct damage is repaired.  However, if the repair is started by returning the vehicle to its original length, the indirect damage may return to its original position.

Figure 6

Metal has memory, and if it is not bent past its elastic point, it will return to its original position — similar to trees in a windstorm (Fig. 5). However, if the metal is bent past this elastic point, the molecules in the metal become deformed and a permanent bend occurs (Fig. 6). At this bend, the metal also becomes stronger or harder from what is called “work hardening,” causing this area to resist

Figure 7

straightening. When the direct and indirect damage are assessed, then access to the damaged areas will determine the repair technique that will be used.

Dent definition

Once the available access is confirmed, the size and definition of the dent should be determined. Some outer areas of the dent may be subtle, and determining the extent of the dented area is critical.

Figure 8

The dent around the gas filler door in Fig. 7, for example, can be determined by a technique called buff grinding. Buff grinding entails using a grinder (with open coat 50 grit, or no coarser than 80 grit if it is aluminum) to remove the paint by holding it at a slight angle, about 1 in. above the surface in areas where the metal is either high or level (Fig. 8). The paint that remains in the dent is low, and must be moved up or back to its original contour. This method is used for both single-side access and double-side access repairs.

Repair technique

Figure 9

Several techniques can be used to repair dents. One method is hammer and dolly, where a dolly (a heavy steel tool with varied shapes, Fig. 9) is chosen that has a shape that closely matches the shape of the part being repaired. The dolly is held on one side — usually the inside — and the hammer is used on the opposite side of the part. Sometimes the hammer blows are placed directly over the stretched metal (hammer-on-dolly, Fig. 10) where the hammer hits a high spot with the dolly's support, thus shrinking or lowering the high spot. In other situations, the hammer blows are placed off to the side of the dolly (hammer-off-dolly, Fig. 11) to raise low areas. As the hammer blows impact the part, the dolly is forced to bounce against the low area, thus raising it.

Pick hammers without dollies are also often used to raise low areas, as are an assortment of other specialty hammers. The head of each is designed in a specific shape to perform specialty tasks, such as a shrinking hammer, which is used to shrink stretched metal without the use of heat.

Being able to place the pick blow precisely where it will have the most effect is a skill that will take some practice. Often, the hammer is held with the index finger on the handle so the blow will be more precise.

Another, even more accurate tool to raise or lower steel is a spring-loaded center punch. In the last stages of metal finishing, when only small areas need to be raised and a very precise point of impact is needed, the spring-loaded center punch is very helpful.

The sequence and placement of hammer and dolly work is also critical. The first four hammer and dolly placements are intended to correct the non-elastic, or work-hardened, damage. After this procedure is done, the remaining five blows will correct the elastic metal. If this sequence is not followed, a “tin can” dent can occur, because the metal has been stretched. Shrinking, whether cold or hot, is more difficult, and should be avoided if possible.

Pick bars such as those used by “Paintless Dent Repair” technicians are also excellent tools with which to raise dents in areas where two-sided access is not available. Though many manufacturers do not recommend drilling holes for access, often a pre-existing access hole can be found, and if so, using a pick bar can be very helpful.

Shrinking is often necessary because of the damage from the collision or from incorrect repair techniques. If the damage area is small, it can often be corrected using the cold method, in which the dolly is held underneath and light spring hammer blows are used on the surface. For more severely stretched metal, heat shrinking must be used. In the past, heat was applied by using a torch; the metal was heated to red-hot, and then spring hammering was done.

The use of a torch is no longer recommended, because the “heat-affect zone” is too large. The best way to apply heat is with an electric stud welder with a shrinking tip. In this way, the heat can be controlled to reach only the metal that is stretched. Once the stretched area is heated, use of the hammer-on-dolly with spring hammering blows, working from the outside inward, will shrink the metal. Quenching with compressed air will cool the area that has been heat shrunk.

Finishing

To finish the metal so no high or low spots exist requires some finessing with light hammer blows — and also by careful feeling of the surface. When imperfections (minor by this time, hopefully) are found, they can be tapped down using light spring hammering. Slapping spoons or slapping files are also helpful during the finishing period. The slapping tools can be used with a dolly underneath to smooth the surface of the metal.

When finishing, the face of each tool being used must be flat and smooth. If these tools have dents, gouges or imperfections on them, the unevenness will transfer to the metal that is being finished. Therefore, if a hammer, as an example, has nicks, it should be filed smooth, and the edge should be beveled and then sanded smooth so it will not transfer any damage to the finish area.

Should all dents be repaired by metal finishing? Probably not. Are there times when metal finishing is the correct choice? Definitely yes. It is a procedure that should be part of good technician’s skill set. The tools needed should be available in the collision shop and should be kept in good repair. With a little practice, technicians can maintain their metal finishing skills and apply them when needed with confidence and success.

SIDEBAR: Corrosion Protection

Anytime a panel is repaired so that both sides are hammered and a dolly is used, a stud welder is used, or heat shrinking has been done, the corrosion protection on both sides will be disturbed — and must be replaced. Follow the manufacturer’s recommendation for corrosion protection application following repair on both sides of the panel. Additionally, restore corrosion protection, even on the access hold if a pick bar was used.

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