Body Repair, in detail

Three materials arrive on the same trailer and want three different repairs. Steel on a cab door or a truck fender work hardens as you pull it, holds a crease, takes heat and welds. Aluminium on a coach sidewall or a box body skin does not shrink back the way steel does, tears at a stretched crease instead of stretching further, and goes brittle if you heat it enough to move it. Composite panels, the FRP and Filon skins on box bodies and RV sidewalls, are a laminate: skin, adhesive, and foam or a backer behind that, so damage travels through the bond rather than staying in the surface.

A dent that takes an hour on a steel fender is therefore a different conversation on an aluminium skin over a stud, and a different one again on a laminated sidewall where the panel has separated from its core. Read the material wrong and the failure gets bought twice. Heating aluminium to pull it leaves a soft area that oil cans in the sun. Filling a laminated panel over a live delamination gives you a smooth wall that bubbles again by the second summer.

• Steel: pull, weld, shrink with controlled heat, section only where the maker permits
• Aluminium: mechanical pull only, and replace stretched skin rather than heat it
• FRP and Filon composite: bond line checked and sounded before any surface work
• Azdel and Luan backers behind an RV skin decide whether the panel can be pushed at all

Modern bodies hold panels on with adhesive as often as with weld, and the two carry a repair completely differently. A welded joint is metallurgical: it wants heat, correct wire and the manufacturer's permitted sectioning point. A bonded joint is chemical: it wants surface preparation, the specified structural adhesive, clamping pressure and cure time, and it is destroyed by grinding away the flange it lives on. Panel bonding on a van sidewall, or an RV skin over a frame stud, is normal engineering rather than a shortcut.

Owners get hurt by a shop that welds a bonded joint, or rivets a joint that was meant to be bonded and sealed. On an aluminium skinned trailer the visible result is a row of fasteners that telegraph through paint inside a season. The invisible result is a seam that now leaks, since a rivet driven through a skin with no butyl or Sikaflex behind it is a water path straight into the wall. Water in a wall is a far more expensive repair than the dent ever was.

Replacement beats repair sooner than most owners expect, and the deciding number is labour rather than the price of the part. A stamped repair panel or a section of skin has a fixed cost. Pulling, filling and sanding a badly creased panel back to a straight profile is open ended, and at $210 per hour a repair that eats twelve hours has quietly become more expensive than a new panel plus the hours to hang it. When an estimate reaches that crossover we say so, partly because the replaced panel is also the better repair.

There is a case running the other way. A discontinued panel on a coach out of production has no price, because it does not exist. Then the honest sequence is repair what is there, or fabricate. We form and bond replacement sections in house for exactly that reason, and on an older coach fabrication is regularly faster and cheaper than chasing a part through salvage yards for six weeks while the vehicle occupies a bay.

• Repair panels and skin sections priced against the labour to straighten what is already there
• Manufacturer sectioning points respected, never invented
• Sections formed and bonded in house where a panel is out of production
• Parts at or under $100 carry a 100 percent markup, parts above that carry 35 percent

A fender and a quarter panel look like the same job and are nothing alike. A fender bolts on. Pull the liner and the lamp, unbolt it, hang the new one, refinish it, and the hardest part is usually the blend into the door. A quarter panel is welded and bonded into the body structure, wraps a wheel arch, carries a fuel filler or a charge port, and on a van it forms part of the sliding door aperture. None of it unbolts.

On a Sprinter, a damaged quarter is a sectioning decision. Full quarter replacement means cutting at factory approved locations, working around the sliding door track and the pillar, then refinishing a very large area running from the door shut all the way back to the tail lamp. Sectioning the lower portion is often correct and much cheaper, but only where the maker permits a cut in that location. Guessing at a cut line on a unibody van is how a repaired vehicle stops being straight, and on anything carrying driver assistance cameras it is also how a calibration stops holding.

Trailer body work concentrates in two places and the same events cause both. Corner caps, the moulded radius pieces where a sidewall meets the front or rear wall, take every low speed contact with a gate post, a tree and a fuel island kerb. Rub rail, the extruded aluminium trim running the length of the sidewall at the joint between panels, takes the sideswipe. Both are sealing components as well as trim, and both are where a travel trailer starts leaking after a scrape nobody thought was serious.

Repair order matters more than material here. The cap comes off, the screw line gets cleaned back to bare surface, old butyl gets removed rather than covered, fastener holes get checked for a spun or stripped stud, and only then does the new cap or rail go on with fresh butyl and sealant. The cheap version of this job is a bead of sealant smeared over the outside of a cracked cap. That holds for one wet season on the coast, after which the wall behind it becomes a subfloor conversation.

Failures in cheap body work follow a short and predictable list. Filler used as a shaping tool at depth, which shrinks and prints a shadow. Rivets or screws driven through a bonded seam without sealant, which telegraph and then leak. A stretched aluminium skin heated to make it move, which oil cans in the first hot week. Bare metal left under primer with no epoxy, which blisters from underneath in coastal air. None of these show on collection day. All of them show inside a year.

Salt deserves its own paragraph. A coach that lives near the water in south Orange County carries a salt film sitting in every seam and every fastener head. Any repair that traps that film under new material corrodes from inside the joint, and the first visible symptom is paint lifting along a seam nobody touched. Washing and neutralising before bonding is unglamorous, cheap, and the difference between a repair that lasts and one that gets rewritten at somebody's expense.

The bottom of the cab body band is a bolt on part and a small refinish: a truck fender, a mirror bracket, a cover on a van. The top is a door skin, a quarter section, a box body panel, and the paint work across three adjacent panels needed to make any of it disappear. What moves a job up is almost never the size of the dent. It is how many pieces must come off first, whether the panel is bonded into structure, and how much surface has to be refinished afterwards.

Two other things push a number up late in the job. Hidden corrosion behind a panel, which nobody can see until the panel is off. And a bracket or mount deformed behind an intact looking skin. Both get photographed and written as a supplement rather than absorbed, because absorbing them means shortcutting one of them, and the shortcut always shows up later on the same panel.