The 1973–1987 C/K trucks are over 35–50 years old now, and frame rust is one of the most important factors when evaluating one for purchase or restoration. Here’s how to assess frame rust on a Square Body and what’s fixable vs what’s a deal-breaker.
Why the Square Body Frame Rusts:
The factory frame rails were minimally protected from the factory – a light paint application over bare steel. In rust belt states (upper Midwest, Northeast), decades of road salt exposure causes significant corrosion.
The Four Stages of Frame Rust:
Stage 1 – Surface Rust:
Red oxide discoloration on the surface, no pitting or metal loss. Treat with rust converter and undercoating. Completely normal and manageable.
Stage 2 – Pitting/Light Scale:
Small pits in the metal surface, some scale buildup. Still structurally sound. Wire brush, treat, and seal.
Stage 3 – Heavy Scale / Wall Thinning:
Metal has significantly lost thickness. Tap with a hammer – if it sounds hollow or crumbles, this is Stage 3. Requires inspection and possibly section repair.
Stage 4 – Rot-Through:
Holes or perforations in the frame rails. This is the deal-breaker zone for most builds. Structural integrity is compromised.
How to Inspect:
- Crawl underneath with a flashlight and a screwdriver
- Poke and prod along the frame rails – especially at the cab mounts, crossmembers, and rear sections
- Check the section from behind the front wheels to the cab mount – this is the most common rot zone
- Look for previous repairs – any filler or patch plates
Treatment Products:
Rust-Oleum Automotive 2-in-1 Rust Reform & Seal Spray – Converts rust and seals in one step. Good for Stage 1–2 rust on accessible frame areas.
Is It Worth Buying?
- Stage 1-2 rust: Yes, manageable with proper treatment
- Stage 3 (some areas): Negotiating point – budget for frame section repair
- Stage 4 (extensive): Walk away unless you’re prepared for a full frame-off restoration
What condition is your Square Body frame in? Share photos if you have them!