Protecting against corrosion during the repair (2023)

For those of us who live in the Northeast or Midwest, no one has to explain corrosion. We have lived with automotive corrosion so long that it registers as specific words in our vocabulary, such as rust bucket or rusty beater.

Protecting against corrosion during the repair (1)

Corrosion is caused by a combination of exposed metal, oxygen and electrolytes. Automobile manufacturers go to great lengths to protect and cover all metal during the manufacturing process. Still, there are many natural causes of corrosion, such as chips and scratches from normal-use road salt applied for winter safety, acid rain (especially in industrial manufacturing areas), polish and moisture of all types. Moisture and its ability to sneak into areas that are not directly exposed to rain or water when washing the vehicle can be difficult to deal with.

In winter when we get into a vehicle with snow on our boots, warm air melts the snow into the floor mat. The heat further evaporates it, allowing the moisture to float into areas where it otherwise would not go, such as inside the dashboard and other places that only vapor can reach.

Protecting against corrosion during the repair (2)
Figure 1 (IMAGE / AL THOMAS )

We also expose metal to the normal repair process as we restore a car to its pre-accident condition. One of our goals when restoring corrosion protection to a vehicle is to completely coat any metal that may have been exposed during the repair process. Often it is quite obvious where our repairs have exposed steel during the repairs (Fig 1).

But other areas are sometimes overlooked, such as the areas of a pinch weld where the anchoring clamp has damaged the factory finish and even through to the corrosion protection, which was applied by the manufacturers (Fig 2).

Protecting against corrosion during the repair (3)
Figure 2 (IMAGE / AL THOMAS )

Even more subtly, when the hammer and dolly method is used to repair steel corrosion repair underneath, either a pick hammer or a dolly (Fig 3) scars the backside of the steel being repaired, and a corrosive hot spot is made. All corrosive hot spots caused by the repair process must have their corrosion protection restored.

Protecting against corrosion during the repair (4)
Figure 3 (IMAGE / AL THOMAS )

As technicians, we must be diligent with not only obvious corrosion protection restoration areas, but even the less evident ones (such as not damaging the vapor barrier between the door and the door trim panel). Or, we may fall short of restoring a vehicle to its pre-accident corrosion resistance.

Figure 4 (IMAGE / AL THOMAS )

Sacrificial corrosion

Corrosion protection materials protect the metal from exposure to the elements. A simple explanation for this protection is that the coating completely covers all the metal, not allowing moisture, oxygen or electrolytes to reach it. However, the process is more complex than just a barrier coat.

Protecting against corrosion during the repair (5)
Figure 5 (IMAGE / AL THOMAS )

The coatings used for corrosion protection, which are sometimes called "direct-to-metal" coatings, contain substances that are more chemically active metals, such as magnesium, zinc, chromium and cadmium, just to name a few.

Protecting against corrosion during the repair (6)
Figure 6 (IMAGE / AL THOMAS )

A compound such as zinc, which is highly chemically active, often is applied to the bare metal at the foundry for use in automotive manufacturing (this process is called galvanization). Also, many paint manufacturers include it in their direct-to-metal corrosion protection coatings. These highly active metals then oxidize quickly, forming a tight bond over all of the exposed metal, which creates a barrier between the metal and the elements that act on it.

Protecting against corrosion during the repair (7)
Figure 7 (IMAGE / AL THOMAS )

This intentional oxidation sacrifices itself for the protection of the metal underneath it. One of the significant factors regarding sacrificial corrosion is its tight bond. It is unlike the corrosion process of rust, which all of us are used to. There, moisture corrodes metal to produce a red flaky substance, which after a while falls off, exposing more bare metal; this corrosion/exposure of new bare metal causes rust holes.

Protecting against corrosion during the repair (8)
Figure 8 (IMAGE / AL THOMAS )

In sacrificial corrosion, once this tight (black) oxidation covers the metal completely, it stays firmly attached and completely eliminates the elements from reaching the metal below.

Protecting against corrosion during the repair (9)
Figure 9a (IMAGE / AL THOMAS )

Corrosion hot spots

During normal repair procedures we cause corrosive hot spots by exposing metal. It may happen during repairs where the backside cannot be accessed and a pulling device is welded on the front (Fig 4), through the impact of a hammer and dolly (Fig 3), due to the attachments of pulling devices (Fig 5), and even when new parts are welded into place (Fig 6). Even pinch clamps and lifting devices (Fig. 7) can cause damage to not only the surface coating, such as the color, but also to even the galvanized sacrificial corrosion that was applied at the factory.

Protecting against corrosion during the repair (10)
Figure 9b (IMAGE / AL THOMAS )

All corrosive hot spots cause damage to the factory-applied corrosion protection, which must be restored after the repairs are finished. It's important that the repairs be finished completely prior to re-applying corrosion protection, because often a repairer needs to use a hammer during the final steps of body repair (Fig 8). If the corrosion protection has been applied prior to this final repair, a corrosive hot spot will exist and may need to have corrosion protection re-applied.

Protecting against corrosion during the repair (11)
Figure 10 (IMAGE / AL THOMAS )

Although it's obvious that a corrosive hot spot has been formed when weld on tools have been used (Figs 9a and 9b), consideration should also be given when tools such as glue-on pulling devices (Figs 10 & 11) have been used. The process of pulling can dislodge the old corrosion protection, and the heat used to remove glue-on tabs could also cause a corrosive hot spot.

Protecting against corrosion during the repair (12)
Figure 11 (IMAGE / AL THOMAS )


Electro-deposition coating, or E-coating, is the method of painting that uses electrical current to deposit the paint. The process works on the principle of magnetism, which charges the paint with a negative charge and the part or vehicle with the opposite charge. The part to be E-coated is then submerged in a bath of paint, which becomes attracted to all surfaces of that part or vehicle.

Figure 12 (IMAGE / AL THOMAS )

The important part to understand is that any surface that is wide enough for the paint to flow into, such as a round spot weld, will have the coating fully applied to it. This means that inside frame rails, between welds, around tight corners and all surfaces are completely coated with this corrosion protection.

Figure 13 (IMAGE / AL THOMAS )

In comparison, spray painting depends heavily on the gun being pointed directly at the part to be coated. Although there are many corrosion protection coatings that can be sprayed on a vehicle after its repair has been completed, it's difficult to match the thoroughness that electro-deposition gives a part at the factory. Because of this, when a repair is planned, exposing the least amount of metal during that repair and conserving the manufacturer's original corrosion protection is a high priority.

Protecting against corrosion during the repair (13)
Figure 14 (IMAGE / AL THOMAS )

What are OEM protections?

  • Galvanization: The first or one of the first protective coatings that is applied is galvanization. This could be applied to the metal as it is being formed and rolled at the foundry; often sheet metal comes to the manufacturer with very specific metal and galvanization requirements. This ensures that the metal can arrive at the factory without corrosion already started; and although it is cleaned prior to stamping and assembly, the majority of the steel has its galvanization intact prior to the automotive manufacturer applying its corrosion protection.
  • Metal treatment: Done at the factory, this is somewhat different from the metal treatment that may be applied in an aftermarket condition. Generally after the vehicle is stamped, welded and partially assembled, it is first cleaned to remove any contaminants that may have been deposited during manufacturing. Then it is sent to a chemical wash area where the metal is cleaned again and slightly etched, so that the electro-deposition, which will be applied next, has a completely clean and prepared surface to adhere to. Metal treatment in the aftermarket application is similar but has some application variations, which I'll discuss later.
  • Primer: Following the E-coat application, automobile bodies have primer surfacer applied. The primer surfacer applied at the factory helps level out any minor imperfections in the body, and it may be tinted relative to the color that will be applied later. The body is inspected, and any imperfections noted are removed by sanding.
  • Chip Coating: In the areas of the vehicle that are prone to have stone chips (such as the leading edge of the hood and lower body parts, including the rockers) chip-resistant coatings are applied. These chip-resistant coatings can be visible, having a higher texture than the rest of the vehicle. This produces a coating that after its application is not noticed, but has the ability to resist impacts from chips and other normal wear during vehicle operation. This chip-resistant coating can either be applied prior to or after the topcoat has been applied; each manufacturer has its own preference.
  • Color coating: This is applied either in a single application (although in recent years single-stage paint from the manufacturer has become nearly extinct) or in a two-stage application process. In the first stage, the technician applies color coat containing primarily color, often along with other glamour features such as metallics or pearls. The surface is then topped with the clearcoat, which has the majority of the protective properties.
  • Clearcoating: The clearcoat, which is applied on top of the color coat, provides protection against ultraviolet rays, acid rain, road salt and other dangers that a vehicle's finish normally encounters during everyday use.

Protecting against corrosion during the repair (14)
Figure 15 (IMAGE / AL THOMAS )

Note: Read, understand, and follow the manufacturer's recommendations. Each manufacturer warranties their vehicles against corrosion. They may warranty for as many as 15 years, covering such things as surface corrosion, rust-through and even panels that have been repaired, as long as those repairing follow the manufacturer's repair recommendations. However, warranties may not cover cosmetic surface corrosion, underbody and body panels not repaired following the manufacturer's recommendation. Technicians should fully understand the manufacturer's required repair procedures and follow them specifically, documenting it in your repair paperwork so that the customer's factory warranty will not be invalidated.

Protecting against corrosion during the repair (15)
Figure 16 (IMAGE / AL THOMAS )

Aftermarket corrosion protection

It's the responsibility of the collision repair facility to safeguard against corrosion and restore the vehicle to its pre-accident corrosion resistance. To do this, one of the most significant precautions that should be taken is to preserve as much original corrosion protection as possible. Each time the original corrosion resistance is disturbed, it's necessary to replace it to the best of our abilities. The easiest way to maintain the vehicle's original corrosion resistance is to not disturb that which was applied at the factory when it all possible. From time to time, a customer may say, "I want it stripped to bare metal." As a conscientious collision repair technician, I realize that when I strip a vehicle to bare metal, it removes all the factory galvanization and leaves the metal completely unprotected – and I therefore must restore it.

Protecting against corrosion during the repair (16)
Figure 17 (IMAGE / AL THOMAS )

Often when removing paint to do repairs, I'll leave as much e-coat on as possible. In fact, when a vehicle has had paint applied multiple times and it's necessary to remove some of the film thickness, it should only be stripped to the factory e-coat. Leaving the e-coat also leaves on corrosion protection. When grinding for welding and other more serious repairs, it helps some if the grinding or sanding can be done using no more aggressive paper than P80 grit. I know it's not always possible to complete the repair without using sanding or grinding equipment more than 80 grit. But doing so will leave intact at least some of the original corrosion protection.

  • Aftermarket Metal Treatment: Although similar to the metal treatment done by the manufacturer, aftermarket metal treatment is applied using three distinct steps. First, the part is thoroughly cleaned and tacked. Then a metal cleaner (Fig 12) is applied. Often it is applied with a squirt bottle, after being mixed to manufacturer's recommendation, and an abrasive pad, used to make sure the cleaner is thoroughly "etched" into the metal (Fig 13). Then, following the manufacturer's recommendation, it is allowed to set prior to using the metal conditioner (Fig 14). Often this conditioner is also scrubbed in, using an abrasive pad (Fig 15).

This method of acid-etch and metal conditioning is considered by most to be the best form of corrosion protection, which can be applied in the aftermarket setting. It is time-consuming and not always necessary, unless the exposed metal area is very large. In most restoration shops, acid etch and metal conditioning is a common practice. Often in the collision repair industry, the application of acid etch or epoxy primer is used instead.

  • Acid etch: (Fig 16) As the name implies, this is a coating that has a phosphoric acid etch agent which, if applied on a properly cleaned and prepared bare metal surface, is an excellent corrosion resistant coating. It is the fastest protection to apply, but cannot be used under body filler. Many of the products can have topcoat applied directly to them. The paint maker's recommendation must be strictly followed; because many acid-etch primers (sometimes called wash primers) can be film-thickness sensitive. Oversaturation may prevent the proper curing of this product. In addition, the paint maker's recommendation for use in enclosed areas should be strictly followed. Many manufacturers do not recommend the use of acid-etch primer on enclosed surfaces, such as frame rails (Fig 17), which do not have sufficient airflow for proper curing.

Protecting against corrosion during the repair (17)
Figure 18 (IMAGE / AL THOMAS )

One of the more convenient products that has arrived in the last few years is acid-etch primer in a spray can. Though more expensive than mixed acid-etch primer (and therefore not suitable for large areas), it is very convenient for covering cut-through and small areas of bare metal that have been exposed during the repair process. Also, it can also be used in enclosed areas for covering corrosive hot spots. It is applied to exposed metal and followed with additional undercoatings.

  • Epoxy primer: Many technicians believe that epoxy primers that are properly mixed and applied provide the closest-to-OEM e-coat protection as can be done in the aftermarket setting. However, the use and application of epoxy primers can be time consuming; some require an induction period, a time after mixing that the product is required to sit prior to spraying. Also, its dry time can be longer than most acid-etch primers. Because of these two factors, often epoxy primers are not used. With a little forethought and planning, though, epoxy primer can be applied near the end of each workday; and it can dry overnight before more work will be done.

  • Weld-through primer: Notice Figures 18 and 19, where weld-through primer corrosion protection has been applied to the repair parts being spliced onto a frame rail. After it is fitted (Fig 20) and welded, additional corrosion protection is applied. Only a small portion of the weld-through primer is destroyed from welding heat. It is also theorized that some of the heated weld-through primer that is not burned away reflows black into the weld site and protects it. Others believe, however, that the use of weld-through primer contaminates the weld and causes it to be less strong. Therefore, caution should be used when using weld-through primers and all manufacturers' recommendations should be read, understood and followed. Some manufacturers may recommend the use of weld-through primers while others do not or may recommend that strict application procedures must be followed.

  • Anti-corrosion compounds: Corrosion protective coatings that fall into this category are generally either petroleum-based or wax-based products. For years, petroleum-based products, often commonly referred to as undercoatings, have been sprayed in areas such as the trunk, inside door compartments and frame rails. Recently these products have been improved, with such characteristics as self-healing when damaged, resistance to water and salts, staying flexible for many years, and the ability to creep into hard to reach areas.

Protecting against corrosion during the repair (18)
Figure 19 (IMAGE / AL THOMAS )

Because they can be sprayed using a spray can or applicator wand that can reach back into cavities, undercoatings have been a primary choice for repaired frame rails and other enclosed surfaces. Recently "cavity waxes," or wax-based corrosion protection products, have become a product of choice for many manufacturers.

Cavity waxes do not require airflow for proper curing and their ability to creep and self-repair helps them reach into areas where they are needed. As new ones appear due to normal wear and operation over a period of years, these cavity waxes creep into these new areas where needed as well.

Protecting against corrosion during the repair (19)
Figure 20 (IMAGE / AL THOMAS )

Restoring a vehicle to its pre-accident condition may sound simple to evaluate – since many people view the success of this procedure by visual and cosmetic observations. But to truly restore a vehicle to its pre-accident condition means that not only do all the cosmetic requirements have to be met, but its corrosion resistance and crashworthiness also must be restored. By not restoring the proper corrosion resistance, one could cause a beautifully repaired vehicle to start to deteriorate almost immediately upon delivery. Restoring a vehicle's corrosion resistance is very important to maintaining long-term satisfied customers.


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