OEMs use E-coat tanks such as this one from TKS Industrial Co. to apply primer to all nooks and crannies. Although impossible to duplicate in the aftermarket, collision repairers have a number of products and OEM procedures to restore corrosion resistance post-repair.Technicians applying cavity wax in a blind cavity should use at least two passes to ensure proper film thickness.3M’s chart shows where to apply anti-corrosive cavity wax in external (shown) and internal crevices, along with typical amounts, such as half a can used for a hood perimeter.3M’s chart shows where to apply anti-corrosive cavity wax in internal (shown) and external crevices, along with typical amounts, such as a full can used for a lower frame rail.This trunk’s bare weld and burned paint, a “hot spot” for corrosion to start, was spotted by Billy Walkowiak, of post-repair inspection company Collision Safety Consultants.A 1-inch bristle disc (shown) or a flattened drill bit can be used to remove weld-through primer in the area to be plug-welded with a MIG/MAG welder (where allowed by the OEM).3M’s 8852 Cavity Wax Plus and the 8851 Cavity Wax Plus Applicator Wand Kit can be used to apply rustproofing in deep nooks and crannies with its 360-degree conical tip.

Post-repair inspections point to deficiencies in corrosion protection

Proper procedures and products can restore OEM protection

Returning a vehicle to “pre-loss condition” is the goal of a technician taking pride in his or her work. Restoring corrosion protection is part of that goal. With a new vehicle body being dipped in a vat of E-Coated primer, duplicating that process is not feasible for collision repairers. But by following the proper procedures, technicians can restore corrosion protection to the repaired area or replacement bolt-on parts.


Corrosion protection is a system, not an estimate line item

Although some may think of “corrosion protection” as only undercoating, it’s actually a number of operations requiring thought and care to ensure long-term durability and performance, including adhesives, primers, seam sealers, “rustproofing,” such as cavity wax, rocker panel “chip guard,” and undercoating.

There is no “one-size-fits-all” solution; one OEM may recommend using weld-through primer, while another may not, instead prescribing resistance spot welding with a sealant or adhesive between the two panels. Collision repair staff must refer to OEM procedures to know what are the proper methods for that vehicle, and the repair methods may differ from how the vehicle was constructed, pointed out 3M Advanced Technical Specialist Shawn Collins at last year’s NACE Automechanika, and who’s been speaking on corrosion protection to trade associations the past year.

As an example, he said, “Many manufacturers are beginning to recommend having a sealer in that joint, even if it wasn’t sealed from the factory,” he said. “You don’t want an adhesive in there unless it’s recommended, because you don’t want to improve the strength in certain areas. But having something in that joint is very important. That will eliminate most of your corrosion problems.”

OEM procedures on which steps and products are required to restore collision protection can be found from AllData, in the OEM repair manual (www.oem1stop.com), or at I-CAR’s Repairability Technical Support portal (www.rts.i-car.com). And ASA’s Not Included Operations list, updated in March and available for download at https://asashop.org/tools-resources/free-industry-tools/, includes product and labor items that must be manually input in estimating systems, such as cavity wax, chip guard, seam sealer and tips, undercoating, and weld-through primer.

I-CAR’s CPS01 classroom course on corrosion protection is recommended by I-CAR for estimators and technicians  to identify “which activities require corrosion protection restoration. Bare metal areas are the obvious hot spot, but disturbed coating or heated areas are also a concern.”

Some procedures have changed relatively recently, pointed out Collins, who is also an I-CAR instructor. Until about five years ago, he said, I-CAR recommended applying an epoxy primer before applying cavity wax.

“But the problem is epoxy, especially in a cavity, takes forever to cure, and then [the technician] ends up spraying cavity wax over uncured epoxy, which causes more problems than it helps. So I-CAR just wants cavity wax sprayed in there without epoxy, because cavity wax is sticky, and it will stick to any soot and burned paint, where epoxy primer will just flake off.”

For procedures using weld-through primer and MIG/MAG welding, Collins cautioned that weld-through primer should be removed in the spot where the plug weld is performed with a one-inch bristle disc or flattened drill bit, as the primer is something of a misnomer — it’s not intended to be welded directly on. And it, and any other primer that’s one-part or etch, should not be sealed over.

“A good rule of thumb is don’t apply seam sealer over any primer the paint manufacturer wouldn’t recommend spraying color over,” he said in a recent Collision Hub Repair University video on the subject. “You can’t spray color directly over etch primer; it does not stick well. And they will not warranty over a 1K primer.”


Deficiencies show in post-repair inspections

As vehicle complexity grows, Collins said he sees the trend of more post-repair inspections growing. Insufficient — or no — corrosion protection is a common deficiency easily uncovered by those companies, he said, either with the naked eye (such as peeling back a trunk weatherstrip) or with a borescope in nooks and crannies. He said quality-control inspections can be done at the shop to catch problems before they leave the shop.

“They’re making them now where you can plug them into your cellphone,” he said. “They’re very common equipment, and I’d highly recommend that shops have those for their own inspections.”


Cavity wax is first line of defense

Although it’s one of the last steps in the repair process — after paint, but before reassembly — cavity wax is the first line of defense in staving off corrosion from moisture and road salt. (For estimators needing assistance justifying its use on an estimate, Mike Anderson has a free “negotiation tool” on his website, collisionadvice.com.)

It should be used not only in welded areas but also in bolt-on parts. 3M has charts at 3m.com showing where, and approximate amounts used, cavity wax should be applied to internal (frame rails, rocker panels, and pillars) and external (fenders, wheel arches, and the perimeters of doors, hoods, and deck lids). As an example, 3M recommends half a can be used on the perimeter of a new hood. Applying inadequate film thickness is a common application error, especially in a blind cavity, such as inside a rocker panel, Collins said, noting that enough should be applied that it drips out of drain holes, placing paper towels on the floor to collect drips.

“You need to make a couple of passes, just to ensure you have complete coverage,” he said. 

Parts & People

Parts & People is published monthly by Automotive Counseling and Publishing Company, Inc., a Colorado corporation, P.O. Box 18731 Denver, CO 80203, 303-765-4664. President-Lance Buchner. Founded by Lance Buchner and Dave Lucia.