Rust Jacking

Rust jacking happens when rust forms between two layers of overlapping sheet metal—like where a quarter panel overlaps the rear body panel, or where an outer rocker panel covers an inner rocker. As the rust grows, it expands with tremendous force (rust occupies more volume than the original steel), literally jacking the two layers of metal apart.

This creates visible bulges, stress cracks in the paint, and eventual structural failure as the metal separates at the seams. The scary part: rust jacking usually starts where you can't see it—inside closed seams and overlapping panels.

By the time you see evidence of rust jacking (paint cracks, panel bulges, visible rust staining), significant damage has already occurred. The repair isn't just treating surface rust—it's cutting out the affected metal and replacing it.

Classic Mustangs are notorious for rust jacking in these areas:

• Lower quarter panels (where outer and inner panels overlap)
• Rocker panels (between inner and outer rockers)
• Cowl area (where windshield frame meets firewall)
• Door bottoms (where outer skin wraps around inner structure)
• Fender lips (where fender overlaps mounting flange)

I learned about rust jacking when a shop showed me what looked like a small bubble in the paint. They ground it down and found that the quarter panel had separated from the inner structure by nearly half an inch. The "small bubble" required cutting out and replacing 18 inches of quarter panel. The repair cost $2,400.

Rust jacking is structurally serious and cosmetically unfixable without metal replacement:

Structural concerns:

• Panel separation - Layers of metal no longer bonded together
• Crack propagation - Stress cracks spread from rust-jacked areas
• Water intrusion - Separated seams allow more water in, accelerating rust
• Weakened structure - Load-bearing areas lose integrity

Cosmetic concerns:

• Visible bulges - Paint can't hide metal that's physically displaced
• Paint cracking - Stress cracks form around rust-jacked seams
• Impossible to blend - Can't body-filler over separated metal
• Progressive failure - Rust jacking spreads if not addressed

The repair reality:

You cannot fix rust jacking with surface treatment or body filler. The only proper repair is cutting out the rust-jacked metal and welding in new panels. This turns "just needs paint" into "needs metalwork" very quickly.

Common Rust Jacking Locations (1964½–1973 Mustangs):

High-Risk Areas:

1. Lower Quarter Panels

1. Rocker Panels

1. Cowl/Windshield Area

1. Door Bottoms

1. Trunk Floor Edges

How to Identify Rust Jacking:

Visual inspection:

• Paint cracks along panel seams
• Bulging or rippling in overlapping areas
• Rust staining seeping from seams
• Uneven gaps between panels
• Visible separation at seam edges

Physical inspection:

• Press on suspected areas (soft or spongy feel)
• Tap with small hammer (dull thud vs. solid ring)
• Check for flex in panels that should be rigid
• Inspect seam edges for visible gaps

Professional inspection:

• Media blasting reveals hidden rust jacking
• Ultrasound thickness testing
• Removal of trim/moldings for hidden areas
• Door jamb and trunk inspection

If you see evidence of rust jacking, assume it's worse than visible. Get a professional assessment before buying or quoting repair.

Prevention (For Clean Cars):

If your Mustang doesn't have rust jacking yet:

• Wash regularly - Remove road salt and dirt from seams
• Inspect seams annually - Catch rust before it spreads
• Keep drain holes clear - Doors, rockers, cowl drains
• Treat chips immediately - Don't let rust start in seams
• Garage storage - Minimize exposure to moisture
• Rust inhibitor spray - Spray into door jambs, rocker seams annually

An ounce of prevention is worth 40 hours of metalwork.