F100 Rust Repair: Common Problem Areas and How to Fix Them

The Rust Problem

Rust is the number one killer of Ford F100 trucks. More trucks have been scrapped due to rust than any mechanical failure. Steel body panels, exposed underbodies, and decades of exposure to moisture, road salt, and trapped debris all take their toll. The good news is that the aftermarket for F100 patch panels and replacement body panels is excellent, and most rust repair is within reach of a motivated home builder with basic welding skills.

The first step is honest assessment. Crawl under the truck, poke at suspect areas with a screwdriver, and figure out what you are actually dealing with. Surface rust is cosmetic. Scale rust is starting to eat into the metal. Penetrating rust has gone all the way through, and structural rust means the metal is no longer doing its job of holding things together.

Common Rust Spots by Generation

All Generations

Some areas rust on every F100 regardless of year. These are the spots to check first:

Cab corners. The lower rear corners of the cab are the single most common rust area on any F100. Water runs down the back of the cab, collects in the corners, and sits there. On many trucks, you can push a finger right through the metal. Replacement cab corner panels are available from LMC Truck, Dennis Carpenter, and National Parts Depot for nearly every generation. They typically cost $30 to $80 per side.

Floor pans. Water enters the cab through worn door seals, leaking windshields, and clogged drain holes, then pools on the floor. Check under the rubber mat or carpet. Floor pan patch panels and full replacement floors are available for most generations. Full floor pans run $150 to $400 depending on the year.

Rocker panels. The rockers sit low and catch road spray, mud, and salt. They rust from the inside out, so by the time you see it on the outside, the damage is usually extensive. Replacement rocker panels run $50 to $150 per side.

Door bottoms. The lower three to four inches of F100 doors are prone to rusting out. Water enters through the window channel, collects inside the door skin, and eats through the bottom. Door bottom patch panels are available, or you can replace the entire door skin.

Early Trucks (1948-1966)

Front fenders. The front fenders on first- and second-generation trucks bolt on, which makes replacement relatively easy. The lower edges and the area behind the headlights are the worst spots. Reproduction fenders are available but are not cheap, running $300 to $800 each depending on the generation.

Bed floor and cross sills. The wood-floored beds on 1948-1956 trucks often have rotted wood and rusted cross sills. The metal strips (cross sills) that support the wood planks rust through. Replacement cross sills and complete bed wood kits are available from suppliers like Mar-K and LMC Truck.

Inner fender wells. Mud and debris pack into the inner fenders and hold moisture against the metal. Check these carefully on any early truck.

Bumpsides and Dentsides (1967-1979)

Bed sides. The lower six inches of the bed sides on these trucks rust aggressively, especially along the wheel arch. Full bed side panels and lower bed side patch panels are widely available.

Behind the rear wheels. The area of the bed side directly behind the rear wheel opening traps road spray. This is one of the first places to go on these trucks.

Inner cab structure. On 1973-1979 trucks, check the inner structure behind the door hinges and the cowl area. Water can enter through deteriorated cowl seals and cause extensive hidden damage.

Late Trucks (1980-1983)

Same as 1973-1979 for the most part, since the 1980-1983 trucks share the same basic body. Pay extra attention to the frame, as these trucks are old enough now to have serious frame rust but new enough that they often get overlooked during inspections.

Repair Methods

Patch Panels

For localized rust, a patch panel is the standard repair. The process is straightforward:

1. Cut out the rusted metal, going at least an inch past the rust in every direction to reach clean metal.
2. Trim the patch panel to fit the opening, leaving about a 1/4-inch overlap or setting up a butt joint.
3. Clamp the patch in place and tack-weld it, then run your final welds.
4. Grind the welds smooth and apply body filler to level the surface.
5. Prime and paint.

A MIG welder is the right tool for this work. A 110V unit will handle thinner body panels (20-gauge and thinner), but a 220V unit gives you more flexibility and better penetration on heavier-gauge metal. Use 0.023-inch or 0.030-inch wire and 75/25 argon/CO2 shielding gas.

Full Panel Replacement

When the rust is too extensive for patching, replacing the entire panel is faster and produces a better result. Full cab corners, floor pans, rocker panels, and bed sides are available as stamped replacement panels that match the original contours. Dennis Carpenter, LMC Truck, and National Parts Depot carry the widest selection.

Full panel replacement involves cutting away the entire damaged section, fitting the new panel, and welding it in. The fit of aftermarket panels varies. Some drop right in; others need persuasion with a hammer and dolly. Always do a test fit before you start welding.

Treating Hidden Rust

After welding in new panels, treat the backside and any enclosed areas with a rust inhibitor like Eastwood Internal Frame Coating, POR-15, or Fluid Film. These products creep into seams and crevices to slow future corrosion. This is especially important inside door skins, rocker panels, and behind cab corners where you cannot easily see or reach.

Prevention

Once you have repaired the rust, protect your work:

• Seal body seams with a quality seam sealer. 3M and Eastwood both make brushable and cartridge-type seam sealers that mimic factory seam treatments.
• Apply undercoating or rubberized undercoat to the floor pans and underbody. Avoid cheap undercoatings that trap moisture. Rubberized or wax-based products are better choices.
• Replace weatherstripping. New door seals, windshield seals, and cowl seals keep water out of the cab. Dennis Carpenter and Precision Replacement Parts carry weatherstripping for most generations.
• Keep drain holes clear. F100 cabs and doors have drain holes that allow water to escape. Make sure these are open and unobstructed.

Parts Sources and Costs

Panel	Estimated Cost
Cab corner patch (per side)	$30-$80
Full floor pan	$150-$400
Rocker panel (per side)	$50-$150
Door bottom patch	$30-$60
Bed floor (steel)	$200-$500
Lower bed side patch	$60-$150
Full front fender (early trucks)	$300-$800

For the best selection, check Dennis Carpenter, LMC Truck, National Parts Depot, and CJ Pony Parts. Mar-K specializes in bed wood and bed hardware. For welding supplies and rust treatment products, Eastwood is a one-stop shop that caters specifically to automotive restoration.

Frequently Asked Questions

How much does F100 rust repair cost?

F100 rust repair costs range from $200 to $3,000 or more depending on severity. Individual patch panels like cab corners run $30 to $80 per side, full floor pans cost $150 to $400, and rocker panels run $50 to $150 per side. A MIG welder and basic body tools are needed for DIY repairs.

Can I do F100 rust repair myself?

Yes, most F100 rust repair is within reach of a home builder with basic MIG welding skills. The process involves cutting out rusted metal, fitting a patch panel or full replacement panel, welding it in, grinding welds smooth, and finishing with body filler, primer, and paint. A 110V MIG welder handles most body panels.

What are the most common rust spots on a Ford F100?

The most common rust areas on every generation F100 are cab corners, floor pans, rocker panels, and door bottoms. Water collects in cab corners and floor pans through worn seals, while rockers rust from the inside out due to road spray. Early trucks also suffer front fender and bed cross-sill rust.

Where can I buy F100 rust repair panels?

The best sources for F100 patch panels and replacement body panels are Dennis Carpenter, LMC Truck, National Parts Depot, and CJ Pony Parts. Mar-K specializes in bed wood and bed hardware. Eastwood carries welding supplies and rust treatment products like internal frame coating and POR-15.

How do I prevent rust from coming back after F100 body repair?

After welding in new panels, treat the backside and enclosed areas with rust inhibitor like POR-15 or Fluid Film. Seal body seams with quality seam sealer, apply rubberized undercoating to floor pans, replace all weatherstripping to keep water out, and make sure all factory drain holes in the cab and doors remain clear.