The ins and outs of corrosion repair

September 1, 2007


Editor’s note: This article is based on a presentation given by Mitch Becker, technical instructor, Abra Auto Body & Glass, Minneapolis, at the 2007 National Auto Glass Conference & Expo. Write him at

The decision to repair rust or corrosion on a vehicle should not be taken lightly. New metals, materials and manufacturing techniques make training an absolute must to ensure techs can properly identify the types of metals and corrosion, use the appropriate products, and understand and implement the correct procedures for repairing or replacing corroded metals and preventing corrosion. To give you an example of the training involved, body shop technicians are required to take a 40-hour lecture course just on metals.

My purpose is not to dissuade auto glass technicians from repairing vehicles with corrosion. Performing such repairs allows technicians to maintain the safety of the vehicle, meet the requirements of the Automotive Glass Replacement Safety Standard, avoid customer complaints and generate additional revenue. However, proper training is crucial.

Aluminum vs. steel
The first step in rust or corrosion repair is to identify the type of base metal you are working with. To do so, refer to the vehicle’s body manual, or use a magnet. If the magnet sticks to the metal, it’s steel. If it doesn’t, it’s more than likely aluminum.

In the past, most vehicle panels were made of steel. Today, manufacturers use new metals and plastics to make vehicles lighter and more fuel-efficient. Aluminum, for  example, is rapidly becoming standard on many vehicle lines, including Honda, Jaguar and Audi. You’ll find aluminum panels in roofs, hoods, doors, quarter panels, tailgates and fenders. Plastics, as used on the 2005 Corvette cowl panel, are also gaining popularity.

In addition, carmakers rely on new manufacturing techniques. Each weld on a vehicle adds weight. As a result, more manufacturers use adhesives to adhere panels to the vehicle body. These bonds are not only stronger than welds, in some cases they enable manufacturers to bond dissimilar metals. For example, the exterior of a door panel could be aluminum and the interior and framing could be magnesium.

This is important because although steel and aluminum have similar repair guidelines, they are “dissimilar” metals. They can’t come in contact with each other. If they do, and an electrolyte, such as water, is introduced, they will corrode at an accelerated rate. If replacing a windshield on an aluminum vehicle, do not use a steel tool. If the steel tool digs into the aluminum, it will transfer steel particles into the aluminum, causing it to corrode. If you use a tool on aluminum, never use it on steel, and vice versa. The exception to this rule is a stainless steel tool, appropriate for use on both types of metal.

The metals issue applies to quarter glasses, door glasses, vent windows and back glasses as well. On the 2007 Chevy Tahoe, the tailgate is aluminum. The quarter panels on Range Rovers are aluminum. If you’re working on an Audi with an aluminum door and you dig your steel tool into the door while replacing the glass, you can contaminate the door metal.

Oxidized steel, rust
Types of corrosion
The second step in rust or corrosion repair is to identify the type of corrosion. There are three types:

• Rust on steel occurs when the base metal is exposed to an electrolyte such as water. The rust penetrates the metal, flakes in layers and reduces metal thickness.

• Aluminum filiform corrosion occurs when the base metal is exposed to an electrolyte due to improper surface preparation. This type of corrosion looks like the paint is flaking off the metal. It’s white, chalky stuff.  Many technicians think they can just wipe it off; that is not the case. This type of corrosion requires surface preparation similar to that of steel.

• Aluminum galvanic corrosion occurs when two dissimilar metals come in contact with one another. If steel and aluminum make contact, the aluminum turns to white powder and the steel starts rusting. In a body shop, if an aluminum vehicle comes in and a technician must expose the bare metal, the vehicle is moved to a separate area away from the steel vehicles to reduce the risk of contamination.

Aluminum filiform corrosion
Repair methods
Incorrect corrosion repairs create an unstable base for auto glass installation and can weaken the metal past the vehicle manufacturer’s recommendations. When removing corrosion, it is extremely important to not damage the structural components of the vehicle, such as welds or adhesive-bonded panels.

Do not grind the rust or corrosion off the vehicle unless absolutely necessary. Grinding thins out the metal quickly and weakens the welds; it does not get deep into the steel’s pores to remove the rust.
Aluminum galvanic corrosion
The heat from the grinding also can destroy alloys in the base metal, causing it to tear or crack. Grinding destroys aluminum instantly. When you grind an aluminum vehicle in one spot, you build up heat and warp the aluminum. In the winter, I have seen technicians use butane torches to heat up the pinchweld to remove moisture. Doing that on an aluminum vehicle will lead to warping.

Excessive heat also can cause body panels to release. Some roof and quarter panels, for example, are bonded to the vehicle with an adhesive. If that adhesive heats up, it releases. Technicians who use induction tools are particularly susceptible to this type of damage to vehicles.

Specialty grinding wheels are a good solution for removing corrosion. They are available for use on steel or aluminum. They remove the corrosion, not the base metal; reduce heat buildup; are available in air or electric versions for mobile use; and are appropriate for corrosion removal on small or large areas.

Drilling, rasp bits or stones are another option. They allow you to focus on areas with rust and do not thin large areas of metal if used correctly. Remember, drill bits and rasps for steel can never be used on aluminum. 

The best method for removing corrosion is sandblasting. It’s a dirty, messy job, but sandblasting guns attack the rust deep down in the pores without thinning the metal as much. They are suitable for use on steel or aluminum and available in portable, handheld versions.  A lot of technicians carry portable sandblasters that allow them to hit a small spot right on the vehicle, clean it up, put the body primer on, wait for it to flash off and then apply a proper pinchweld primer. It’s a quick and easy process.

Although there are chemicals available for rust removal, they have never been tested to work with adhesive manufacturers’ primers. I don’t recommend their use.

Once you’ve removed the corrosion, clean the metal according to the car or paint manufacturer’s recommendations. Protect existing urethane outside of the repair area against contamination.

The use of incorrect primers, or the incorrect application of such primers, can accelerate the corrosion’s return. Make sure to use the appropriate primers. If your adhesive manufacturer has a primer for bare metal, great; if not, apply a body primer to protect the metal. Be aware that some manufacturers require special primers, especially on aluminum vehicles.

Don’t apply primers in too thick or thin a layer. What happens when you paint a house and put one thick layer on to avoid
having to do multiple coats? It peels off. The same is true of primer; it has to be applied in multiple coats. Allow the primer time to cure between those coats.

I know a lot of body shops will paint and prime the pinchweld and call an auto glass technician an hour later to come put the windshield in. If the body primer has not had time to cure, it will lift up when you apply the pinchweld primer, contaminating the entire job.

For primer application, follow manufacturer guidelines regarding the use of safety equipment, cure times and temperatures. If you spray primers on the pinchweld, be aware this type of air application involves the use of respirators and other safety precautions. 
Seal scratches, whether on aluminum or steel.

Body shop issues
When doing a windshield installation on a vehicle in a body shop, make sure the technician has used the correct metal primer. Make sure there are no body fillers or foams in the pinchweld. How many times have you gone to a body shop and the technician has put body filler in the pinchweld where he’s replaced the roof? That needs to come out.

Make sure there are no seam sealers, or smear, unless recommended by the vehicle manufacturer. On Saturn vehicles, for example, the roof is an adhesive-bonded panel. Sometimes the body shop technician will smear the squeeze-out across the pinchweld. That has to come off. You don’t know if your urethane or primer will bond to it.

In addition, make sure the body shop technician has not painted the pinchweld. Paint or clear coat in the pinchweld is not acceptable as it does not attain car manufacturers’ required adhesion levels for urethane bonding. In a crash, that paint is your weak link.

If the vehicle is not repairable due to holes, missing metal or previous body work, notify the owner and have it towed to the professional body shop.What if?
What if the vehicle is too far gone? When you take the windshield out, are you liable for that installation? You need to address this with your company and lawyer. At my company, I’m not liable until I put the windshield in. If I take the windshield out and find the vehicle is not repairable, I’m not liable for that installation.

If the vehicle is not repairable due to holes, missing metal or previous bodywork, notify the owner and have it towed to a professional body shop. Keep the tow bill as proof that the owner was notified of the damage and the dangers associated with installing glass over it. This decision should be the company owner’s or lawyer’s responsibility, not the technician’s. A plan of action should be in place for technicians and office personnel to follow. Have flyers on hand for customers that explain rust issues, as well as a list of recommended body shops.

If you take an OEM windshield out and discover aluminum filiform corrosion underneath that windshield, contact the owner of the vehicle and advise them to contact the dealer. In that case, aluminum filiform corrosion is a warranty issue. The only way it could occur was if the manufacturer improperly prepped the metal underneath. That’s not your problem; it’s the dealer’s problem.

If you discover rust on a vehicle that has come into your shop for a glass replacement, note it on all paperwork and notify the customer. Be aware that no auto repair shop will warranty rust repair, and even if you do a rust repair, there’s no guarantee the rust or corrosion won’t come back.

Corrosion destroys auto glass bonding to metal. It also destroys the metal bonding to the inner structure of the vehicle, weakening the metal and causing the panel to fail. As auto glass technicians, we don’t want to create a weak link in the installation chain. Safety of the customer is paramount. Do people’s lives depend on our ability to do the job correctly? Absolutely.

Training resources
• Adhesive manufacturers
• I-CAR,
• National Institute for Automotive Excellence,
• National Glass Association,