Restoring Corrosion Protection During Vehicle Repair

Introduction

Corrosion protection has become a very important step in vehicle repair due to the greater number of unitized automobiles on the road today. These automobiles are made of thinner high strength steels to reduce the weight of structural members and exterior body panels. To protect this thin high strength steel from corrosion, the factory uses state of the art corrosion protective processes, coatings and sealers.

Factory corrosion protection which is disturbed during a collision or during the repair process, MUST be restored to a similar level as the undamaged portion of the vehicle being repaired, or a future corrosion problem may develop. Through the proper use of quality corrosion preventive products now available to the repair industry, the potential for a future corrosion problem can be minimized. If quality corrosion preventive products are applied while using proper repair techniques, a sound corrosion resistant repair can be achieved.

General Guidelines for Corrosion Protection

Corrosion protection coatings can be damaged from cutting panels and seams either mechanically or with heat, minor straightening and stress relieving procedures.

Always check other surrounding areas other than the damaged area for any cracked seams, damaged corrosion protective sealers or coatings, then repair as necessary.

Always use the appropriate respiratory and eye protection along with following any other necessary safety precautions during the collision repair process.

Stay with one paint manufacturer's paint system throughout the repair.

DO NOT mix product brands during a repair. Each product is designed to work with other products of the same manufacturer's paint system.

ALWAYS READ AND FOLLOW ALL INSTRUCTIONS CAREFULLY about product purpose, application procedures and compatibilities with other products, provided by the specific product manufacturer.

Always use a quality weld-through zinc rich coating on any non-galvanized mating surfaces of joints and seams prior to welding. The amount of corrosion formation, between two adjoining panels at joints or seams, can be significantly reduced with the use of a quality weld-through coating.

While dressing down any cosmetic welds also dress down welds which have pits in the weld metal surface. The first place where corrosion starts is in pitted welds.

DO NOT use lacquer based primers on bare metal.

Before applying any primer, always prepare the surface as directed by the paint manufacturer.

Use either self-etching or epoxy primers as the first layer of primer on ALL bare metal surfaces (both internal and external) to obtain maximum paint adhesion, after preparing the surface as directed by the paint manufacturer.

Use the appropriate type of body caulking/seam sealer to seal joints where gaps are present. Also, seal all flange and overlap joints and seams to seal out air, water and other corrosive environmental contaminants.

Use a paint sealer over primer, when it is included as part of a paint manufacturer's paint system, before applying the color coat.

Always use a waxy type of anti-corrosion compound, which is light-bodied and designed to migrate between metal-to-metal surfaces, on joints and seams, inside closed sections, and on other surfaces. This type of anti-corrosion material should be used on all other areas which do not have direct environmental exposure or exposure to road abrasion. The use of conventional rubberized or tar based undercoating should be limited to the vehicle's underbody and wheelhouse areas, only.

Repair Steps and Reasons for Performing Them

1. Removing Primer From Service Panels

The primer applied to service panels, by the factory, must be removed in the areas to be welded prior to welding service panels in place. The primer must be removed from both mating surfaces of the joint or seam. The remainder of the factory primer should be scuff sanded prior to priming.

Some galvanized service panels are also primed. When removing the primer in the appropriate areas, care should be taken to remove only the primer, leaving the galvanized layer.

Some suggested methods are:

(a) 3M "Scotch-Brite" Surface Conditioning Disc (coarse grade disc on a soft holder) or the Clean'N Strip Disc

(b) Propane Torch and Wire Brush (power or hand)

(c) Abrasive Paper on D/A (80 grit)

2. Zinc Rich Weld-Through Coating

A quality weld-through coating should be applied to all nongalvanized mating surfaces to be welded. Then apply the weld-through coating as directed, to the two surfaces to be joined while taking care not to apply coating beyond 1/4" (6 mm) past the joint or seam contact area. After the welding operation is complete, the excess or oversprayed weld-through coating should be removed by sanding.

3. Gas Metal Arc Welding (G.M.A.W.)
(Formerly referred to as: Metallic Inert Gas, M.I.G. Welding)

Prior to performing a welding operation on the vehicle, a practice test weld, of the same type as being utilized, must be performed, on like scrap material and conditions, to test for proper weld penetration. A test weld of good quality, both visually and physically, will confirm the welder is properly adjusted for the particular welding situation.

NOTE:
The welder may need to be readjusted when welding joints or seams where a weld-through coating has been applied.

4. Cleaning And Dressing Welds After Welding

Properly cleaning ALL welds after welding is one of the most important steps in preventing corrosion formation. The first place in which corrosion will start is at weld locations. This is due to the metal oxidation and slag formed at weld locations during the welding process. Paint will not stick to oxidized metal or the slag which forms on the weld metal.

Two methods for cleaning welds are:

(a) Abrasive blasting is a very effective method in accessible areas and when equipment is available.

(b) Power wire brushing is another effective method which is inexpensive, fast and is approximately as effective as abrasive blasting. It also removes less of the galvanized layer in the surrounding area.

After cleaning, it is important to dress down (grind flush) cosmetic welds, especially those welds containing pits, which are difficult to properly clean. Weld pits are prime starting places for corrosion.

A jet of compressed air should always be used after cleaning and dressing welded areas. It will remove most residue in limited access areas and remove any loose particles from the weld areas. Then use a wax and grease removing solvent to clean all welded areas prior to applying the base primer, following the manufacturer's surface preparation instructions.

5. Wax and Grease Removing Solvent

Do NOT wipe surface with paint solvents. They just smear the dirt leaving a residue which interferes with proper paint adhesion. Wax and grease removing solvents are designed to dissolve and float off greasy dirt. Apply it liberally while keeping the surface wet until all grease and dirt is completely dissolved. Then wipe dry with a clean white cloth.

*The most important part of good corrosion prevention procedures is a clean surface. A clean surface means completely free of all foreign matter such as dirt, grease, oil, wax, oxidation, grinding or welding residue and finally chemically cleaned.

6. Preparing Bare Metal Prior to Applying The First Coat of Primer

When using most two component self-etching primers, metal conditioner and conversion coating are not required prior to its application. Although it may not be required, metal conditioner can be used on all bare metal surfaces to enhance the primers adhesion capabilities. When using most two component epoxy primers, both metal conditioner and conversion coating are required for obtaining good paint adhesion.

Regardless of which type of primer is used, do not use metal conditioners or conversion coatings on or near seams and closed sections. Some metal conditioners and most conversion coatings require a water rinse step, so care should be taken to prevent water from being trapped inside seams or closed sections. Surface preparation procedures will vary, so be sure to follow the specific surface preparation procedures supplied by the paint manufacturer.

A. Metal Conditioner

Metal conditioning bare metal is chemically cleaning the metal surface. Metal conditioners are usually a phosphoric acid based cleaner which chemically cleans by etching the bare metal. Etching creates a surface finish which the base primer can adhere to easily. (Excluding bare metal which has been abrasive blasted). When using metal conditioner, be sure to follow product manufacturer's application instructions carefully for proper results.

Do not touch bare metal with your hands after surface has been treated with metal conditioner. Doing so will contaminate the clean surface with oils from your skin.

B. Conversion Coating

Conversion coating is used to form a uniform, chemically bonded zinc phosphate layer on the metal surface. The zinc phosphate layer is used to form a chemical structure which facilitates primer adhesion. When rinsing the surface, check for surface cleanliness by observing that the water sheets out and does not bead or channel.

7. Priming Bare Metal (Using Corrosion Inhibiting Primers)

*The first primer coat applied over bare metal is a very important step in corrosion protection. The base primer coat must form a secure bond to the bare metal and provide a good surface for the subsequent paint layers to adhere.

To obtain the best paint adhesion and protection against corrosion: A two component Self-Etching Primer containing phosphoric acid or a two component Epoxy Primer should be used as a first coat over bare metal after it is properly prepared as directed by the product manufacturer.

*Do NOT use a lacquer based primer as a first coat over bare metal. The lacquer based primers do not protect against corrosion nearly as well as the self-etching or epoxy type primers, due to poor adhesion and porosity of the lacquer based primer.

8. Body Caulking/Seam Sealer

*The surface should be primed before and after applying most types of caulking and sealer compounds.

The proper use of body caulking/seam sealer is very important in vehicle corrosion protection. By filling gaps and sealing seams, the vulnerable edges of body joints and seams will be sealed from air, water and other corrosive environmental contaminants. Seam sealer will also keep out dust and exhaust fumes, along with reducing road noise.

9. High Build Primer

Be sure to read and follow instructions on base primer and high build primer to check on compatibilities and application procedures. Use a quality primer surfacer which is compatible with the base primer, paint sealer and color coat to be used.

10. Paint Sealer

Paint sealer is applied over the last primer coat prior to the application of the color coat. Some primers are porous and require the use of a paint sealer. The paint sealer is designed to create a barrier layer to prevent paint solvents and moisture from soaking into the porous primer, therefore aiding in corrosion prevention. Paint sealer is not required over all primers, although it can be beneficial to use over any primer.

11. Color Coat

The type of color coat used, must be compatible with the specific paint manufacturer's paint system being used during the repair. Stay with one paint manufacturer's paint system for best results.

12. Anti-Corrosion Compounds

There are various types of anti-corrosion compounds available to the repair industry today. Different products are designed for different purposes and should be used only for their intended application.

The primary purpose of anti-corrosion compounds is to provide a barrier layer to seal the painted metal surface from air, water and other corrosive environmental contaminants. Since different areas of a vehicle are exposed to different environments, they need different products to protect against these different environments.

There are basically two types of anti-corrosion compounds.

(a) "Undercoating Materials" are petroleum/tar based, anti-corrosion compounds which are primarily designed to withstand environmental exposure and road abrasion. Quality undercoatings should resist cracking, chipping and washing off. They should be used on the vehicle underbody and wheelhouse areas. This type of material also acts as a sound deadener.

(b) "Rust Proofing Materials" are waxy, anti-corrosion compounds which are designed to spread and creep into cracks and are generally NOT used in road abrasion areas. They are designed for use inside closed sections, inside fenders, doors, quarter panels and in other similar areas. A pressure pot and assorted spray wands will be required to gain access and properly coat the appropriate areas during application.

*For the best results when applying anti-corrosion compounds, especially the rust proofing types, be sure that the material is thoroughly warm before attempting its application. The most important aspect of properly applying any anti-corrosion compound is obtaining complete surface coverage. Warming the anti-corrosion material in a bucket of hot tap water prior to application will provide for better material atomization, therefore a much better chance of obtaining complete coverage of the desired surface.

General Steps to Follow for Corrosion Protection

The entire body structure of a vehicle can be broken down into six types of areas. Each of these areas of a vehicle does not experience the same corrosion environment. It is not practical to protect each from corrosion in the same manner. These areas are determined by: the type and extent of corrosion protection needed, when in the repair process the area needs to be protected against corrosion, and the accessibility of the area.

These areas and their respective sequence of corrosion protection steps will be presented in the same order which they will typically be performed during the repair process. This order may vary depending upon the extent of damage being repaired. Therefore, all work should first be performed on the innermost, structural areas of the vehicle. Then work outward to the vehicle exterior. Priming and painting operations should be performed in the same manner. Anti-corrosion compounds should be applied after all adjacent areas have been color coated to prevent contamination of adjacent bare or primed surfaces. The inner structure of the vehicle should be primed, painted and protected with anti-corrosion material prior to assembling and outer sheet metal on the vehicle.

Joints and Seams

This category includes joints and seams throughout the vehicle which are only accessible prior to welding the seam together. The desired objectives are corrosion protection, and joint and seam sealing.

Sequence of Corrosion Protection Steps to Follow:

1. Clean with wax and grease removing solvent.
2. Dry with clean white cloth before solvent dries.
3. Remove paint from areas to be welded.
4. Apply a quality Weld-Through Coating as directed by the product manufacturer.
5. Perform the necessary welding operations, after fabricating and testing sample welds.
6. Clean all welds with an abrasive blaster or a power wire brush to remove the welding slag and residue.
7. Dress down cosmetic welds and any pitted welds, at least enough to remove the weld pits. (Reclean welds if necessary.)
8. Sand surrounding area as required.
9. Direct a jet of compressed air into and around all welded areas.
10. Apply either a two component Self-Etching Primer containing phosphoric acid or a two component Epoxy Primer, after cleaning and preparing the metal as directed by the paint manufacturer.
11. Apply the appropriate types of Seam Sealer/Body Caulking to seal all seams and fill gaps as required.
12. Reprime areas where seam sealer/body caulking have been applied.
13. Apply a Paint Sealer. (optional)
14. Apply color coat.
15. Apply the Rust Proofing type of anti-corrosion compound. (Use the undercoating type of anti-corrosion compound on the joint or seam it if is exposed to underbody road environment and abrasion.)

Inside Closed Sections

This category includes all types of enclosed surfaces which are generally not accessible. These areas are only accessible with special equipment or prior to installation of the particular component. These areas include the inside of rockers, rails, reinforcements and any other closed sections. The desired objective is primarily corrosion protection.

Sequence of Corrosion Protection Steps to Follow:

1. Prior to installing a service part which forms a closed section, clean the entire part with a Wax and Grease Removing Solvent.
2. Dry with clean white cloth before solvent dries.
3. After all pre-welding anti-corrosion steps, welding procedures and post welding cleaning procedures have been completed (as described in steps #3-8 in the section on JOINTS AND SEAMS), direct a jet of compressed air into and around the closed section to remove any grinding or welding residue.
4. Using a pressure pot and a 360 degree spray wand, start with the innermost area working outward, flood the inside of the closed section with either a two component Self-Etching Primer or a two component Epoxy Primer.
5. After the outside of the closed section is prepared and painted (as described in steps #10-14 in the section on JOINTS AND SEAMS), proceed as directed.
6. Using a pressure pot and various spray wands to suit the application, apply the Rust Proofing type of anti-corrosion compound to the inside of the closed section, again starting with the innermost area.

Back Sides of Exterior Panels

This category includes areas which may be accessible to installation or with special equipment after installation such as inner surfaces of doors, fenders and quarter panels, etc. The desired objective is corrosion protection.

Sequence of Corrosion Protection Steps to Follow:

1. Clean with Wax and Grease Removing Solvent.
2. Dry with clean white cloth before solvent dries.
3. Scuff sand the surface.
4. Where bare metal is exposed, clean and prepare metal as directed by the paint manufacturer and apply a self-etching or two component Epoxy Primer. (For maximum corrosion protection, apply the Epoxy Primer even if there is no bare metal exposed.)
5. After the panel installation is complete and the vehicle exterior has been color coated, apply the Rust Proofing type of anti-corrosion compound. Application will require a pressure pot and assorted spray wands to obtain complete coverage.

Semi-Cosmetic Areas

This category includes areas that have varied contours which result in poor accessibility for sanding and surface preparation such as: engine compartment, fender and door edges, core support, inner decklid, door jams, etc. The desired objectives are corrosion protection, paint sealing and good paint shine.

Sequence of Corrosion Protection Steps to Follow:

1. Clean with Wax and Grease Removing Solvent.
2. Dry with clean white cloth before solvent dries.
3. Scuff sand the surface.
4. Apply either a two component Self-Etching Primer containing phosphoric acid or a two component Epoxy Primer, after cleaning and preparing the surface as directed by the paint manufacturer.
5. Apply the appropriate types of Seam Sealer/Body Caulking to seal all seams and fill gaps as required.
6. Reprime areas where seam sealer/body caulking have been applied.
7. Apply a Paint Sealer. (optional)
8. Apply color coat.

Exterior Cosmetic Areas

This category includes areas which are easily accessible to sand and prepare for painting such as the exterior surface of a vehicle. The desired objectives are good corrosion protection, paint sealing, smooth surface and paint shine.

Sequence of Corrosion Protection Steps to Follow:

1. Clean with Wax and Grease Removing Solvent.
2. Dry with clean white cloth before solvent dries.
3. Sand surface as required.
4. Apply either a two component Self-Etching Primer containing phosphoric acid or a two component Epoxy Primer, after cleaning and preparing the surface as directed by the paint manufacturer.
5. Apply a quality high build Primer Surfacer after preparing the surface as directed, while following the manufacturer's procedures to obtain the desired surface quality.
   NOTE: Water with a high mineral content (such as well water) will leave mineral deposits on the primer surface as it dries. These deposits are contaminants and will leave marks or may cause the paint to raise.
6. Apply a Paint Sealer. (If included in the paint manufacturer's paint system.)
7. Apply color coat.

Vehicle Underbody

This category includes the areas which are difficult to sand or prepare and are exposed to the severe road environment such as the complete underbody and wheel housings. The desired objectives are corrosion protection and good joint and seam sealing.

Sequence of Corrosion Protection Steps to Follow:

1. Clean with Wax and Grease Removing Solvent.
2. Dry with clean cloth before solvent dries.
3. Sand as required.
4. Apply either a two component Self-Etching Primer containing phosphoric acid or a two component Epoxy Primer, after cleaning and preparing the surface as directed by the paint manufacturer.
5. Apply the appropriate types of Seam Sealer/Body Caulking to seal all seams and fill gaps as required.
6. Reprime areas where seam sealer/body caulking have been applied.
7. Apply color coat.
8. Using a pressure pot and assorted spray wands, apply the Under-coating type of anti-corrosion compound to the entire underbody of the vehicle and wheelhouse areas. Do not undercoat the brake lines, fuel lines, exhaust system or the drive train components.

In conclusion to all the information presented, there is not anything new or revolutionary required to properly restore the corrosion protection during vehicle repair. The only extra steps are to properly apply a quality corrosion inhibiting primer and rust proofing compound to the inside of closed sections.

To briefly summarize, the three most important points to follow when restoring corrosion protection during a repair are:

1. Carefully read and follow all paint/product manufacturer's instructions about product usage and application procedures.

2. Properly clean and prepare all bare metal surfaces prior to applying the base primer coat.

3. Always use a quality corrosion preventive primer as the base primer coat on bare metal. Either a self-etching or epoxy primer, or an equivalent primer will be required to obtain a similar level of corrosion protection as the factory primers.

Therefore, by using quality corrosion preventive products and proper repair techniques, you can achieve a sound corrosion resistant repair.

The information provided in this bulletin is for educational purposes only. Although every effort has been made to ensure the accuracy of the information contained in this bulletin, Tech-Cor assumes no responsibility or liability for any repairs performed using information from any publication issued by Tech-Cor.

Any person performing repairs must determine whether any suggested or recommended procedures or repairs are suitable or appropriate for the particular vehicle being repaired. The repairer remains solely responsible for such determination, as well as for the proper completion of the repairs.

Reproduction of this bulletin is not permitted without the written approval of Tech-Cor.