# After Ten Years on Route, Pull the Rocker Trim and Look Behind It > On a high mileage service body the rust you can see is the smallest part. What hides under rub rails, how to test for perforation, and when a patch stops making sense. Source: https://ocrv.info/blog/fleet-rocker-corrosion-service-body Published 2026-08-24. Category: Commercial and Fleet Maintenance. About 6 minutes. Corrosion on a service body concentrates where water sits and cannot escape: the base of vertical compartment corners, the rocker under the compartment floor, the body to subframe seam, and plugged drains. Rub rails and trim conceal it for years. Remove the trim and probe the metal before deciding between a patch and a section. ## The short answer Rust on a working service body does not start where you can see it. It starts at the bottom of vertical compartment corners, in the rocker under the compartment floor, along the seam where the body sits on the subframe, and at every drain that filled with road grit. Rub rails and stainless trim hide the evidence for years while holding moisture against the paint. Take the trim off, probe with an awl, and decide on what the metal does under pressure. ## Why corrosion starts at corners and rockers Water needs somewhere to sit and time to sit there. A service body offers both in the same handful of places every time. Vertical compartment corners are folded flanges meeting a floor pan. Sealer runs thin at the crease during assembly, and the remaining capillary gap wicks water in and holds it after the surface has dried. The rocker below the compartment floor catches everything the front tires throw rearward and sits in the least ventilated air on the vehicle. The seam where the body lands on the subframe often includes a wood or composite spacer that behaves like a sponge with a paint job over it. Dissimilar metals accelerate all of it. An aluminum body bolted to a steel subframe with plated steel fasteners is a galvanic cell every time the joint gets wet, and the corrosion attacks the aluminum around the fastener rather than the fastener itself. Then there is the trim. Rub rails and bright strips are screwed straight through the finish. Every screw is a breach in the coating, and the rail itself holds a wet poultice of road film against the panel through every wash and every rain. Trim that was installed to protect the body is one of the more reliable ways to destroy it. ## What you find when the trim comes off Paint blistering pushed up from behind, in a line that follows the rail rather than any impact. Blisters that break open under a fingernail and reveal dark, damp scale instead of clean primer. Laminated scale is the next surprise. Oxide occupies several times the volume of the steel it consumed, so it builds in layers and jacks whatever sits on top of it outward. If a rub rail has been standing proud of the body on one end, that is why, and nobody bent it. Fasteners will fight you. Screws seized in their holes, heads that round off under a driver, machine screws that shear when you back them out. Budget removal time for the trim and its hardware separately from the metal work, because pulling forty fasteners out of a corroded flange is its own job. Undercoating adds a twist. A coating that cracked years ago stops being a barrier and starts being a moisture trap, sealing water against bare steel and holding it there through dry weather. Sound deadening and any insulation behind a compartment wall does the same thing. The outside of that panel frequently looks acceptable the whole time. Corrosion works from the back, where sealer never reached and nobody ever looked. ## Surface scale, perforation, and the line between them A visual inspection always underestimates, because the metal loss is on the side you cannot see. Four tests give you a real answer. - **Magnet.** Sound steel grabs firmly. Heavy scale weakens the pull. An area that has been filled gives you nothing. Fast, and useless on aluminum, but it sorts a panel quickly. - **Pick test.** Push an awl into the suspect area with moderate hand pressure, the amount you would use to mark a hole. If it sinks in, that is perforation and the metal there is finished. - **Tap test.** A small hammer on sound panel rings. On a thinned panel it thuds, and the pitch drop is audible before the awl confirms it. - **Ultrasonic thickness gauge.** Take a reference reading on a known sound area of the same panel first, then compare. This is the only test that quantifies what is left. The line is straightforward once you have the data. Scale that cleans back to bright metal without measurable thickness loss is a coating repair: mechanical removal, conversion treatment, epoxy primer, seal. Metal that has lost section or holed under a pick is a metal repair, and no amount of filler changes that. Aluminum bodies read differently. Look for white powder at fastener heads, pitting under a chalky surface, and filiform corrosion tracking out from the edges of scratches in worm-like lines under the paint film. ## Cut and patch versus full rocker section replacement Three questions decide it: how far the perforation runs, whether the substrate behind it is sound, and whether the seam carries load. Cut and patch is appropriate when the affected area is local, the flange the patch will land on is solid, and the joint is skin rather than structure. Cut back to a clean geometric shape well past the corrosion, form a patch from the same gauge, butt weld it with backing rather than lapping it, and stitch in short sequences so the panel does not distort. Dress the welds, prime both faces with epoxy, seam seal the joint, and get cavity wax into the boxed section through an access hole before you plug it. Patching only the outer face and leaving the back bare means you will see the same panel again. Full section replacement is the call when the perforation runs past a body mount, when the flange the patch would attach to is itself gone, or when you would be welding patches onto patches. Replace out to a factory joint or a fabricated one you choose deliberately, and reproduce the original drain provision in the new section. That work is [welding and metal fabrication](/services/custom-fabrication-and-detailing/welding-and-metal-fabrication) rather than panel repair, and it is scoped the same way as a [structural panel replacement](/services/body-glass-and-doors/panel-replacement) on any body. ## Compartment floors and the drains that stopped draining Every compartment floor is pitched toward a drain, and every drain on a truck with a decade of route work is a candidate for being plugged. Road grit packs the tube. A shop rag gets pushed into the corner and stays. Spilled sealer or spray adhesive glazes the opening. A rubber mat laid in for convenience covers the hole entirely. Test with a hose after any repair. Pour a gallon into each compartment and watch. Water should be gone in seconds. If it pools and sits, the drain is wrong or the pitch is wrong, and the repair you just finished has an expiration date. Adding a drain is straightforward if you do it correctly. Find the true low point with a level while the truck sits on flat ground, not by eye. Confirm nothing structural sits underneath before you cut, because a drain drilled into the top of a subframe crossmember drains onto steel and stays there. Roll or flare the edge, then coat the cut edge with primer and sealer, since a bare drilled hole is simply the next corrosion origin with a better view. Watch where the water lands, too. A compartment drain that empties into a boxed rocker has not solved anything. ## Deciding whether a high mileage body earns the repair Frame it as remaining service years, not as a dollar figure in isolation. A sound chassis under a tired body is a genuine fork in the road, because swapping a used or new body onto that chassis is a real option with its own costs: mounting hardware, wiring, lighting, finish work, and the downtime to do it. Repair earns its keep when corrosion is confined to two or three identified areas, the subframe and body mounts are solid, and the chassis has real years ahead of it. That is a bounded job you can scope from photos and probe results. Reconsider when the body to subframe seam is compromised in multiple bays, when compartment floors are perforated across the length of the body, or when the unit is already inside its last replacement cycle. Spending body labor at $210/hr on a body scheduled for retirement is money that belongs in the next spec instead. Either way, document before disassembly. Photograph each area, record probe and gauge findings by location, and keep the set with the unit. The [repair range estimator](/tools/repair-range-estimator) is a reasonable way to bracket the conversation before a truck ever makes the drive from Eastvale, and the [posted rate structure](/prices) shows which categories the hours will land in. --- OCRV Center. 23281 La Palma Ave, Yorba Linda, CA 92887. (949) 799-3387. info@ocrvcenter.com. Serving Eastvale, California in Riverside County, about 20 miles from the shop. All work is performed in shop at the Yorba Linda facility. No mobile, roadside or on-site service is offered. Scope is body, paint, structural, fiberglass, interior and vehicle systems work. Engine rebuilds, transmission rebuilds, drivetrain work, DOT inspections and emission testing are not performed.