The first step in treating rust is keeping it from forming, which means keeping your truck's metal painted, plated, or otherwise protected so oxidation can't get a foothold. If it's already too late and you see those tell-tale bubbles under the paint or surface rust spreading across a panel or framerail, you've got to take immediate action. Once rust starts, it'll keep growing until your sheetmetal looks like that really old Swiss cheese you found buried in the fridge behind last week's meatloaf.
How you deal with your rust problem depends on how badly the rust has damaged the metal. If the condition is serious enough to have poked holes and/or compromised the metal's integrity, then surgery-cutting away the affected metal and replacing it-is the only recourse you have. If the rust is just superficial and the metal is still sound, then you can remove or neutralize the rust with mechanical or chemical treatments, and then treat the area to keep rust from forming again. Here's a quick rundown of each of these strategies.
Cut it Out
We'll start with the worst case. If you're seeing daylight through the metal, the rust is bad enough that the affected area should be trimmed away and replaced. Anything from a grinding wheel or cutting disc to a plasma cutter or torch can be used to do the cutting. If you're using heat, be sure to stay clear of upholstery, wiring, fuel lines, and other components that could be damaged or catch fire. Don't just cut out the rust; cut about 11/42 inch or more beyond the obvious damage so that you're left with uncontaminated metal to work on.
Speaking of welding, opinions vary as to which weld type is best for this kind of repair. John Morrow of Johns Customz & Performance, an automotive restoration and performance shop, prefers MIG-welding as it's easier for him to control the heat in the weld. Jim McSorley, head fabricator at Land Cruiser specialty house TLC, chooses TIG welding since the TIG weld is easier on a metal finish. Adds TLC's Jonathan Ward, "Be sure you match the [TIG] filler rod to the same kind of metal you're welding, so the welded area remains as pliable as a virgin panel."
Not all rust is obvious. TLC's Jim McSorley knows that the rear sills on FJ40s tend to rust since they're sandwiched behind body panels and collect moisture and debris. This '83 FJ40 body tub has already been media blasted; now McSorley cuts away the lower part of the driver-side corner panel to get at what he believes is a rusted sill end. | 
He was right. Since this portion of the sill mounts a spare-tire carrier bracket, McSorley will cut out and patch the rusted metal before replacing the corner panel... | 
...The passenger-side sill end doesn't carry weight, and it's invisible behind the corner panel, so McSorley just treated the rusty part with a coating of POR-15. |
Patch panels can be either butt- or flange-welded to the repair area. The flange weld, where the new piece overlaps the old, is generally stronger, though you'll have to smooth the overlap area to get rid of its visible seam. Morrow recommends putting the patch underneath the original panel so you'll have a smaller area to cover with body filler. A butt weld won't have that kind of overlap, but you will have to carefully trim the patch panel so that it's just slightly smaller than the hole, and then be careful to fill the gap with weld. Morrow feels that butt-welding is best done by experienced welders only.
If the damage to the panel is extensive enough, it may be easier to replace it than patch it. The good news is that plenty of companies offer reproduction floors, tailgates, fenders, and other body parts. Some even specialize in particular makes, like James Duff for Broncos; Heritage Sheet Metal for Land Cruisers; and Year One, Classic Industries, and Goodmark Industries for Chevy and GMC trucks. (Those last three deal primarily in musclecar restorations, but each has pickup parts in its inventory.)
Here's the patch panel McSorley made to go over the POR-15'd sill. He trimmed it from a larger panel made for FJ40s by Heritage Sheet Metal. You want to keep the repair area as small as possible to reduce the risk of metal distortion from welding heat. Note that the panel will be flanged at the top so it overlaps with the tub when it's welded in place. That ensures a strong bond. | 
McSorley stitch-welds the patch panel in place with a TIG welder, being careful to work around different parts of the weld area, and not work in a continuous line, to avoid heat buildup in any one place. He used cleco clips to hold the panel in place, "but that's just me being geeky," he said. Clamps or vise-grips would work too. | 
One of the main reasons McSorley prefers TIG welding is the fact that the weld area can be metal finished with a hammer and dolly. A MIG-weld would generally be too hard "and the hammer would bounce right off it," he said. |
Factory Rust Prevention. In Other Words, Do You Need the $100 Undercoating?
Jerry
"That undercoating, that's a rip-off, isn't it, David?"
David, a car salesman
"Oh, we don't even know what it is."
Rust-prevention talk hit the mainstream when Seinfeld took a jab at it in an episode called "The Dealership." And it's the question that will not die: Do you need to buy the extra protection? To get the answer, we hit up Toyota, GM, and DaimlerChrysler for the scoop on what happens at the factory to eliminate modern-day rust-buckets.
Back in the '60s and '70s, safer driving in winter became a national obsession-at least in the northeastern U.S. and the southeastern part of Canada. In fact, Canada created the Canadian Corrosion Code (now obsolete), which required a corrosion check to be part of the vehicle inspection. Even some U.S. states experimented with such a code. Vehicles did indeed leave the factory back then with some rust protection-but not a lot. Most of the sheetmetal was bare steel with some areas hosed down in dip primers or Zincrometal coating, and there was wax in cavities. But joints weren't covered to prevent water intrusion, and junk could build up throughout the vehicle because of poor drainage, eventually causing corrosion. Yet keep in mind that this was back when vehicle life expectancy wasn't intended to be much beyond 5 years.
But then automakers switched steels. Rather than building vehicles with untreated mild-steel exterior panels, they teamed up with the steel suppliers, paint suppliers, and other suppliers to develop steel with the rust-fighter already on it before the body panel even was stamped out. The result was a precoated steel sheet-specifically, galvanized (hot zinc-dipped) and galvannealed (cold zinc-coated). Zinc has the power to heal thyself; scratch or ding the paint, and the zinc will "grow" to protect the steel base material beneath the surface. While automakers already were using some galvanized steel prior to this, that version was more difficult to paint and weld, so its application tended to be hard-to-see places.
Immersion paint systems (phosphate formulas) also were utilized, as was an electrodeposition coat (EDC or e-coat, which is primer that covers every inch of steel panel, done through an electrical-current "bath"). This ensured that the body cavities were painted (since it wasn't so easy to get to those through spray primer). Automotive paints and clearcoats also improved, becoming more advanced at resisting scratches. Heck, even the emblems and exterior trim switched to being attached via adhesive rather than locating pins that would make holes in the panels.
Vehicle design improved too, such as with smaller gaps between panels, watertight seams, and fitted, nonmetal bumper covers. Automakers also ensured that the paint wasn't cut at any point during the build, and that dissimilar metals weren't being joined in such a way that there'd be a bad galvanic interaction. The good galvanic interaction that keeps galvanized steel from corroding can go awry when the wrong metals are mated. Some of the larger cast-iron underbody chassis pieces are left untreated, but any surface corrosion that makes its way onto them is simply cosmetic, nothing that will affect performance.
One more thing: It hasn't been just the factory that's helping the cause. A lot less road salt is being used today. Earth-friendly products have replaced that mineral, and they're much kinder to paint. Even street sweepers are contributing. The paint on fenders, rocker panels, and bumpers is positioned to get pretty abused from dust, sand, and other particles, but that parking ticket you received between 8 and 10 a.m. on Tuesday was worth it in the long run since the streets are cleaner for your truck.
But back to that undercoating. Skip it-it really won't add any value to your truck. Does all this factory prep mean it'll never rust? We would never say never, but it has a good chance of living a long, cancer-free life.
-Tori Tellem