Corrosion Protection To Structural Part Interiors

Figure 1 - Anti-corrosion compound is applied to enclosed interior surfaces with a wand using access holes.

There has been a longstanding recommendation to apply epoxy primer, as well as anti-corrosion compound, on the inside of rails and pillars and rocker panels as a last step for structural repairs. Going back as far as the July/August 1988 I-CAR Advantage, in the article "Restoring Corrosion Protection," is the following step for providing corrosion protection to enclosed interior surfaces: "Apply primer. Two-part epoxy recommended. Then apply anti-corrosion compound." The reason given, is that on areas where the coatings have been entirely removed, this is a two-step process that is replacing the two original coatings, zinc and E-coat.

When research for the recently updated I-CAR corrosion protection course, I-CAR asked several product and vehicle makers if this is still the most frequent recommendation. I-CAR was told it is not, due to several reasons. These include possible primer adhesion problems on these surfaces, the lower prevalence of epoxy primer at repair facilities, the increased popularity of self-etching primer, changing primer chemistries, and an increase in the effectiveness of anti-corrosion compound. E-coat is the best corrosion protection material that will ever be applied to a vehicle surface, and aside of the weld backside, the enclosed interior areas have E-coat.

Still, there is a concern among repair facilities for longevity of repairs, retaining corrosion warranties, and assurance that there is one more layer of protection, especially in the rust-belt areas. For these reasons, and more, several facilities will continue to apply epoxy primer in addition to anti-corrosion compound to enclosed surfaces as a standard operating procedure.

Primer Requires a Cleaned Surface
Product makers have a concern with adhesion if these backside areas are not cleaned properly. Any surface to be primed must be cleaned, both mechanically and chemically. If applying epoxy primer inside rails and other structural parts, the surfaces must be cleaned before the part is assembled. This requires sanding followed by a thorough chemical cleaning, especially the joint area backside to remove coatings. There might be access to a rail joint, if the joint is near the very tip of the rail and the rail is open-ended up through the joint location. Other than a situation like that, there isn't enough access for proper cleaning after welding, so it must be cleaned before assembly.

Anti-Corrosion Compound Alone
The recommendation from the product makers and vehicle makers that I-CAR communicated with during research for updating the I-CAR corrosion protection course is to apply anti-corrosion compound alone. Anti-corrosion compound, applied as a mist coat using a wand inserted through access holes or from the end of a rail, is designed to protect enclosed areas from corrosion (see Figure 1). The proper application technique should treat the backsides of the welds. Excess material should creep into the flange areas, sealing off those areas from moisture. At least one vehicle maker, Chrysler LLC (aka FCA/Stellantis), recommends a double application of anti-corrosion compound (see Video).

Video: Chrysler Collision Repair Manager Doug Craig explains that Chrysler recommends applying two coats of anti-corrosion compound.

Do Not Use Self-Etching Primer

It is still recommended to not use self-etching primer inside enclosed areas, because it doesn't form a barrier like epoxy primer and would eventually break down. Self-etching primer must be coated with another product, and it would be difficult to see inside an enclosed area whether or not the primer is completely coated by the anti-corrosion compound.

The recommendation to apply epoxy primer followed by anti-corrosion compound inside an enclosed area is not included in the updated I-CAR corrosion protection course. When using this two-step process, the surfaces must be cleaned for the primer to properly adhere. The more general recommendation is applying anti-corrosion compound alone.

This article first appeared in the August 4, 2011 edition of the I-CAR Advantage Online.

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