Cab corner rust on 1988–1998 GMT400 trucks is extremely common. Whether to patch or fully replace depends on the extent of the rust. Here’s how to decide.
Assess the Damage First:
Before buying any parts, probe the rust with a screwdriver or pick:
- Soft, flaky rust that pokes through: Full replacement needed
- Surface scale with no perforation: Patch repair is fine
- Rust within 2" of good metal: Patch works
- Rust extending into the B-pillar or door jamb: Full section needed
Option 1: Patch Panel (Mild to Moderate Rust)
Small to medium rust areas can be cut out and patched with sheet metal or reproduction patch panels. Requires welding skills or a MIG welder.
Option 2: Full Reproduction Cab Corner Panel
Reproduction full cab corner panels are available for the GMT400. These replace the entire outer skin of the cab corner from door to rocker.
Option 3: No-Weld Patch (Temporary)
For trucks you’re selling or as a stopgap, panel bonding adhesive and body filler can cover small rust areas. Not recommended for long-term structural use.
Recommended Treatment Before Closing Up:
Rust-Oleum 2-in-1 Rust Reform & Seal – Apply inside the repaired cavity before welding to prevent rust recurrence.
Why Does the GMT400 Rust Here?
The cab corner panel has a multi-layer construction with a seam at the lower edge. This seam traps moisture and debris, especially in areas where the factory seam sealer has cracked or failed.
Preventing Future Rust:
After any repair:
- Apply seam sealer to all new joints
- Apply inner body cavity wax
- Paint the inner structure before closing up
- Annual inspection of cab corners and wheel arches
Has your GMT400 experienced cab corner rust? Share your repair approach!