Rust Repair and Prevention for 1973-1987 GM Square Body Trucks
Overview
Rust repair and prevention is one of the most critical maintenance aspects for 1973-1987 GM Square Body trucks (C10, C20, K10, K20, K5 Blazer, Suburban). These vehicles are now 35-50 years old, and rust has become the primary threat to their survival. GM's factory rust protection was minimal by today's standards, and decades of exposure to road salt, moisture, and environmental elements have taken their toll.
This comprehensive guide covers identification, repair techniques, and prevention strategies for all common rust areas on Square Body trucks. Whether you're dealing with surface rust or complete panel replacement, proper rust remediation is essential for preserving these classic trucks for future generations.
Why This Matters:
- Rust spreads exponentially if left untreated
- Structural integrity depends on solid metal
- Proper repair prevents costly future problems
- Maintains vehicle value and safety
Common Issues
Primary Rust Areas by Priority
Critical Structural Areas:
- Cab corners - Universal problem on all Square Bodies
- Rocker panels - Hidden rust spreads behind trim
- Door frames - Bottom of door openings
- Firewall area - Battery acid corrosion (driver side)
- Frame rails - Especially rear sections on long beds
Common Body Panel Issues:
- Fender wells - Front and rear wheel openings
- Bed floors - 1973-1987 all models
- Tailgate bottoms - Water collection area
- Hood edges - Particularly front edge
- Windshield frame - Lower corners
Model-Specific Problems:
- K-Series 4WD: Transfer case crossmember area
- Blazer/Suburban: Rear quarter panels behind wheels
- Long bed trucks: Rear frame section flex cracking
Rust Progression Stages
- Surface rust - Orange discoloration, treatable with conversion
- Scale rust - Flaking, requires mechanical removal
- Penetrating rust - Holes forming, needs patch work
- Structural rust - Complete panel replacement required
Parts & Materials Needed
Replacement Panels
Major Structural Components:
Cab Corners:
- LMC Truck: 34-1473 (Left), 34-1474 (Right)
- Classic Industries: K1034L/R
- Goodmark: GMK4143473L/R
Rocker Panels:
- LMC Truck: 34-1469 (Inner), 34-1470 (Outer)
- Brothers Truck Parts: BTP-ROC-7387
Door Frames:
- LMC Truck: 34-1475 (Driver), 34-1476 (Passenger)
Bed Components:
Bed Floors:
- Short bed: LMC 34-2473 (1973-1987)
- Long bed: LMC 34-2474 (1973-1987)
Bed Sides:
- LMC Truck: 34-2469 (Left), 34-2470 (Right)
- Specify short bed (6.5') or long bed (8')
Tools Required
Cutting Tools:
- 4" angle grinder with cutting wheels
- Reciprocating saw with metal blades
- Aviation snips (straight and curved)
- Plasma cutter (professional option)
Welding Equipment:
- MIG welder (minimum 140-amp)
- .023" or .030" ER70S-6 wire
- C25 shielding gas (75% Argon, 25% CO2)
- Welding clamps and magnets
Preparation Tools:
- Wire brushes (manual and rotary)
- Sandpaper: 80, 120, 220, 400 grit
- DA sander with discs
- Needle scalers for tight areas
Rust Treatment Materials
Chemical Treatments:
- Corroseal Water-Based Rust Converter - Best overall performance
- SEM Metal Prep - Professional grade phosphoric acid
- Ospho - Hardware store option for light rust
- POR-15 - Complete three-step system
Primers and Coatings:
- SEM Self-Etching Primer - Direct to metal application
- Rust Mort - Neutralizes remaining rust traces
- 3M Cavity Wax - Interior frame protection
- Fluid Film - Annual undercoating maintenance
Patch Materials
Sheet Metal:
- 18-gauge cold-rolled steel for structural repairs
- 20-gauge for non-structural patches
- Pre-cut patch panels from LMC or Classic Industries
Fasteners:
Plug Welds: 1/4" holes, 1-2" spacing
Seam Sealer: 3M 08509 or SEM 29512
Body Filler: Evercoat Rage or Bondo Professional
Step-by-Step Guide
Phase 1: Assessment and Planning
1. Complete Inspection
- Remove all trim pieces and rubber seals
- Pressure wash undercarriage to reveal hidden rust
- Use probe tool to test suspected soft areas
- Document with photos for reference
2. Categorize Damage
- Surface treatment areas - Wire brush and convert
- Patch repair zones - Cut out and weld patches
- Panel replacement sections - Complete component swap
Phase 2: Surface Rust Treatment
1. Mechanical Removal
Tools: Wire brush, 80-grit sandpaper, angle grinder
Process:
- Remove all loose rust and scale
- Sand to bright metal in 2" border around rust
- Clean with acetone or metal prep solvent
- Work in 2'x2' sections to prevent flash rusting
2. Chemical Conversion
Application Process:
1. Apply rust converter with brush (not spray)
2. Work temperature: 50-85°F, low humidity preferred
3. Allow 2-4 hours cure time (turns black when ready)
4. Light sand with 220-grit before priming
5. Prime within 24 hours of conversion
Phase 3: Patch Panel Installation
1. Template Creation
- Use cardboard to create precise templates
- Add 1/2" overlap on all edges for welding
- Transfer template to 18-gauge steel
- Cut patch with aviation snips or plasma
2. Preparation
Cut-out Process:
- Mark cut lines 1/2" inside rust boundary
- Use reciprocating saw for straight cuts
- Angle grinder for curves and detail work
- File edges smooth and check patch fit
- Clean both surfaces with wire brush
3. Welding Process
MIG Settings (140-amp welder):
- Voltage: 17-19 volts
- Wire Speed: 200-250 IPM
- .023" wire recommended for thin metal
Technique:
- Tack weld corners first
- Skip weld pattern to prevent warping
- 1/2" welds with 1" gaps, then fill gaps
- Grind welds flush when cool
Phase 4: Major Panel Replacement
1. Cab Corner Replacement (Most Common)
Preparation:
- Support cab with jack stands at frame points
- Remove door for better access
- Cut old corner 1" into good metal
- Clean attachment points thoroughly
Installation Process:
Fitment:
1. Trial fit new panel with clamps
2. Adjust door gaps before final welding
3. Tack weld top and bottom first
4. Check door operation between tacks
5. Complete welding in skip pattern
Critical Measurements:
- Door gap: 3/16" uniform top to bottom
- Flush fit with adjacent panels
- Maintain original body lines
2. Rocker Panel Replacement
Access Requirements:
- Remove doors and door sills
- Remove interior kick panels
- Support vehicle on frame, not rockers
Installation Notes:
Inner/Outer Panel Coordination:
- Replace inner panel first if needed
- Outer panel overlaps inner by 1/2"
- Use original plug weld locations
- Maintain structural box section integrity
Phase 5: Protection and Finishing
1. Primer Application
Surface Preparation:
- 220-grit sand all repair areas
- Clean with tack cloth
- Mask surrounding areas
Primer Process:
- Apply self-etching primer in light coats
- 2-3 coats with 10-minute flash time
- Sand with 400-grit before base coat
2. Interior Protection
Frame and Hidden Areas:
- Spray 3M Cavity Wax into frame rails
- Apply through existing drain holes
- Use extension wand for complete coverage
- Annual reapplication recommended
Year/Model Differences
Early vs. Late Square Body Differences
1973-1980 Models:
- Thinner body metal (more rust-prone)
- Different door handle mounting (affects door frame repairs)
- Single-stage paint system (easier color matching)
- Frame differences in K-series transfer case mounts
1981-1987 Models:
- Slightly thicker metal in some areas
- Improved factory undercoating (minimal difference)
- Base coat/clear coat paint system
- Revised door handle design affects patch panel fit
Model-Specific Considerations
C10/C20 2WD Trucks:
- Lower ride height increases road salt exposure
- Frame rail rust typically at rear spring mounts
- Easier access for repairs (no transfer case)
K10/K20 4WD Trucks:
- Transfer case area collects debris and moisture
- Front differential housing creates splash patterns
- Frame reinforcement at transfer case mount point
K5 Blazer:
- Removable top creates unique water entry points
- Rear quarter panel rust behind spare tire
- Tailgate hinge area particularly vulnerable
Suburban:
- Extended rear overhang increases frame flex
- Rear door area additional rust concern
- Longer rocker panels require sectional approach
Bed Length Variations
Short Bed (6.5 feet):
- Single bed floor panel available
- More rigid structure, less flex cracking
- Cab corner panels same as long bed
Long Bed (8 feet):
- Two-piece bed floor construction
- Frame flex creates stress cracks
- Rear frame section most vulnerable to rust
Tips & Best Practices
Professional Techniques
Welding Best Practices:
- Back-purge critical welds - Use argon gas behind weld for cleaner penetration
- Stitch welding prevents warping - Never weld continuously on thin metal
- Copper backing strips - Prevent blow-through on edge welds
- Heat management - Allow cooling between weld sections
Surface Preparation Excellence:
Professional Standards:
- Clean metal should show uniform scratch pattern
- No contamination from oils or old undercoating
- Feather edge all paint transitions minimum 4"
- Use tack cloth immediately before primer
Cost-Effective Approaches
Budget Repair Methods:
- Por-15 system - Good for DIY surface treatment
- Cut patches from donor vehicles - Salvage yard sourcing
- Sectional repairs - Replace only damaged portions
- Annual maintenance - Fluid Film treatment prevents progression
Premium Approaches:
- Complete panel replacement - LMC or Brothers Truck Parts
- Media blasting - Professional rust removal
- Epoxy primer systems - Maximum corrosion protection
- Professional paint matching - Concours-level finish
Prevention Strategies
Annual Maintenance Schedule:
Spring (Post-Winter):
- Pressure wash undercarriage thoroughly
- Inspect for new rust development
- Touch up any compromised areas
- Apply Fluid Film to frame and suspension
Fall (Pre-Winter):
- Heavy undercoating application
- Seal any new penetrations
- Treat surface rust immediately
- Protect battery area from acid corrosion
Long-term Protection:
- Garage storage reduces rust progression by 75%
- Avoid road salt when possible during winter months
- Regular washing especially wheel wells and frame
- Prompt repair of any paint damage or scratches
Common Mistakes to Avoid
Preparation Errors:
- Insufficient rust removal - Rust will continue under new metal
- Contaminated surfaces - Oil or grease prevents proper adhesion
- Wrong primer choice - Not all primers work on treated rust
- Rushing cure times - Chemical processes need full time
Welding Problems:
- Continuous welding - Causes warping and burn-through
- Wrong wire type - Use ER70S-6 for automotive applications
- Improper fit-up - Gaps over 1/8" require backing material
- No heat management - Allow cooling between weld passes
Protection Oversights:
- Incomplete coverage - Missing areas will rust first
- Wrong product selection - Match product to environment
- No maintenance plan - One-time treatment insufficient
- Ignoring hidden areas - Frame rails and box sections critical
Troubleshooting
Rust Treatment Failures
Problem: Rust Returns After Treatment
Diagnosis:
- Check for incomplete mechanical removal
- Verify chemical converter was properly cured
- Look for moisture infiltration sources
- Test primer adhesion with tape test
Solutions:
- Strip to bare metal and restart process
- Seal water entry points (drain holes, seams)
- Use better rust converter (Corroseal vs. Ospho)
- Apply epoxy primer for maximum protection
Problem: Paint Not Adhering to Repairs
Common Causes:
- Contamination from oils or wax
- Incompatible primer/paint combination
- Insufficient surface preparation
- Moisture in air lines (spray application)
Corrections:
- Clean with acetone before any coating
- Use manufacturer-matched primer systems
- Sand to uniform 400-grit finish
- Install moisture trap in air system
Welding Issues
Problem: Burn-Through on Thin Metal
Prevention:
- Use .023" wire instead of .030"
- Reduce voltage settings 10-15%
- Back with copper strip or backing plate
- Stitch weld rather than continuous bead
Repair:
- Grind out burned area completely
- Cut backing patch 1" larger than hole
- Clamp backing patch behind repair
- Reweld with proper technique
Problem: Warping During Panel Installation
Causes:
- Excessive heat input
- Welding sequence incorrect
- Insufficient clamping/fixturing
- Continuous welding technique
Prevention:
- Skip weld pattern (1/2" on, 1" off)
- Alternate sides of panel during welding
- Use heat sinks (wet rags) on adjacent areas
- Clamp panel firmly before welding
Panel Fit Issues
Problem: Door Gaps Incorrect After Cab Corner Replacement
Diagnosis Process:
1. Check door hinge pin wear
2. Measure gap at multiple points
3. Verify striker plate alignment
4. Test door operation through full swing
Corrections:
- Adjust hinges before assuming panel problem
- Use door gap gauges for precise measurement
- Modify panel mounting points if necessary
- Professional alignment may be required
Problem: New Panel Doesn't Match Body Lines
Quality Issues:
- Aftermarket panels may not match exactly
- Original body may have been damaged previously
- Panel may be for wrong year range
Solutions:
- Verify correct part number for year/model
- Use original panel as template when possible
- Professional metalwork may be required for perfect fit
- Consider multiple suppliers for better fit
Long-term Durability Problems
Problem: Repairs Failing Prematurely
Analysis:
- Document environmental conditions
- Check for ongoing water infiltration
- Verify maintenance schedule compliance
- Inspect for galvanic corrosion (dissimilar metals)
Prevention:
- Use proper drainage design
- Apply appropriate sealers to all joints
- Maintain annual protection schedule
- Isolate steel repairs from aluminum components
This comprehensive approach to rust repair and prevention will help ensure your Square Body truck survives for future generations. Remember that rust repair is often a multi-stage process, and patience during preparation and curing phases is essential for long-term success.