Maintaining and applying metal finishing skills

With some practice, technicians can maintain metal finishing skills and apply them when needed with confidence and success

Al Thomas ABRN auto body repair collision repair metal finishing In efficient collision repair shops, parts replacement often is the preferred method. Consequently, precise and efficient metal repair may not be considered as often.

Metal finishing – the art of restoring damaged metal to its pre-accident condition with little or no filler – has become a less-practiced art. Some argue it may become a lost art soon. Others say the thinner, stronger metals don't finish as easily as the older, softer and thicker ones do. They're correct. Try to bump high-strength steel compared to, say, an older 60s fender. The older metal moves much differently than the newer metal, but it doesn't mean the repair can't be done on high-strength surfaces. In many areas outside North America, repair, not replacement, is the preferred method.

This isn't to say repair should always be the chosen method. One of the critical decisions 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 replacement is the best choice. On the other hand, the art of metal finishing, done well and efficiently, should neither be lost nor avoided. It's a valuable skill new and old technicians should be capable of performing efficiently. Keeping a few tips in mind will help you remain confident and successful when metal finishing.

Plan of attack

Like most repair processes, metal finishing should start with a plan. Inspect the damaged area. Determine what part is direct damage (caused by impact) and what part is indirect damage (caused by the deforming of the panel from the impact).

In Fig. 1, the impact in front of the vehicle shortened it, causing the indirect (sometimes called secondary) damage in the roof. Indirect damage often is larger in area than direct damage. If the repair is started in the wrong area, more damage can be caused than existed initially. For example, in the damage in Fig. 1, if the repair is started in the roof area, it would be almost 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.

Metal has memory, and if it's not bent past its elastic point, it will return to its original position – similar to trees in a wind storm (Fig. 2). However, if the metal is bent past the elastic point, the molecules in the metal become deformed and a permanent bend occurs (Fig. 3). At this bend, the metal also becomes stronger or harder from work hardening, causing the area to resist straightening. When direct and indirect damages are assessed, access to the damaged areas will determine what repair technique will be used.

Access

If a technician can access both sides of the damaged area (Fig. 4), methods such as hammer-and-dolly repairs are used. If only one side can be accessed, other techniques, such as stud weld (Fig. 5) or the glue-on method (Fig. 6), are used. There are numerous single-side repair methods, such as the weld-on wire with progressive pull technique and the use of a suction cup.

Dent definition

Once access is determined, the size of 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. The dent around the gas filler door in Fig. 7) can be determined by a technique called buff grinding, which entails using a grinder (with open coat 50 grit, or no coarser than 80 grit if it's aluminum) to remove the paint by holding it at a slight angle, about one inch above the surface in back. The grinder is moved right and left over the dent, removing paint in the areas where the metal is 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 single-side and double-side access repairs.

Repair techniques

Once the dent is defined and paint is removed, choose a repair technique. If both sides of the damaged area are open, the hammer-and-dolly method can be used (Fig. 9). The dolly is held under the low areas, and the hammer is struck directly above the dolly (Fig. 10) or slightly off the dolly in the hammer-off-dollay method (Fig. 11). The dolly is held so the hammer strikes a high spot. The dolly rebounds on the other side, raising the low area. Be careful not to hammer too hard, especially when using the hammer-on-dolly technique, because the metal may stretch, causing an "oil can dent" or a stretched area to occur (Fig. 12)

Low areas also may be raised by using the pick method, in which a pick hammer is used under the low area without a dolly above it (Fig. 13). Care also should be taken when using this particular method. If too much power is used, overcorrecting or additional damage may be caused. Aiming will be difficult when using a pick hammer from below or on the side of the panel that can't be seen. Hammer blows placed in the incorrect place will cause unwanted damage that will need to be corrected before the metal finishing is complete. Practice is necessary for the technician to become as accurate as possible. Holding the hammer as shown in Fig. 14 will help. The index finger acts as a guide, and because it's a familiar position, aiming accuracy is increased.

The sequence and placement of hammer-and-dolly work also is critical, as seen in Fig. 15. The first four hammer-and-dolly placements are intended to correct the nonelastic or work-hardened damage. After this procedure is completed, the remaining five blows will correct the elastic metal. If this sequence is not followed exactly, a "tin can dent" can occur because the metal has been stretched. Shrinking, whether cold or hot, is much more difficult and should be avoided if it is possible.

Pick bars, such as those used by paintless dent repair technicians, are excellent tools to raise dents in areas where two-sided access isn't available. Though many manufacturers don't recommend drilling holes for access, a preexisting access hole often can be found. If so, using a pick bar (Fig. 16) may help.

Finishing

Finishing the metal so no high or low spots exist requires finessing with light hammer blows and feeling of the surface carefully. When imperfections (minor by this time, hopefully) are found, they can be tapped down using light spring hammering (Fig. 17. Slapping spoons or slapping files (Fig. 18) also are 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 any nicks, it should be filed smooth (Fig. 19), and the edge should be beveled and then sanded smooth (Fig. 20) so it won't transfer any damage to the finish area.

Shrinking

Shrinking often is necessary because of the damage from the collision or from incorrect repair techniques. If the damage area is small, it often can be corrected using the cold method, as seen in Fig. 9, 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 (Fig. 21) so the heat can be controlled to reach only the metal that's 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.

Corrosion protection

Any time a panel is repaired so both sides are hammered, a dolly and stud welder are 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.

Using the skill

All dents probably should not be repaired by metal finishing, but there are certain times when metal finishing is the appropriate choice. It's 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.