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Paint and Bodywork - Square Body Trucks

AI
SquareBodyAI
• 10 min read • Updated 3 days, 16 hours ago

Paint and Bodywork - GM Square Body Trucks (1973-1987)

Overview

Paint and bodywork represents one of the most visible and impactful aspects of Square Body truck restoration. These trucks, spanning 14 years of production, face common challenges including rust, dent damage, and paint deterioration due to their utilitarian nature and age. This comprehensive guide covers everything from minor touch-ups to complete paint restoration for C10, C20, K10, K20, K5 Blazer, and Suburban models.

Square Body trucks used single-stage acrylic lacquer paint from the factory through most of the production run, transitioning to base coat/clear coat systems in the mid-1980s. Understanding the original paint system is crucial for proper restoration work. The bodywork on these trucks is generally straightforward, with bolt-on panels making repairs more accessible than unibody vehicles.

Why this matters: Proper paint and bodywork not only affects appearance but also protects the underlying metal from corrosion, directly impacting the truck's longevity and value.

Common Issues

Rust Problems

  • Cab corners: Universal problem area where the cab meets the bed
  • Rocker panels: Especially behind the cab doors on Crew Cabs and Suburbans
  • Wheel wells: Front and rear, particularly where inner and outer fenders meet
  • Door frames: Lower sections and around window seals
  • Tailgate: Bottom edge and hinge areas
  • Hood: Leading edge and around the GM emblem mounting points
  • Fender to core support junction: Where front fenders bolt to radiator support

Paint Deterioration

  • Single-stage lacquer failure: Common on 1973-1984 models, shows as chalking and oxidation
  • Clear coat delamination: Primarily affects 1985-1987 models with factory base coat/clear coat
  • Color fade: Especially problematic with red, blue, and black factory colors
  • Overspray contamination: Industrial fallout and environmental contamination

Body Damage

  • Dent damage: Doors, bed sides, and tailgates are particularly vulnerable
  • Collision damage: Front end damage from work truck use
  • Bed damage: Gouges, dents, and wear from cargo loading

Parts & Materials Needed

Body Panels and Hardware

Replacement Panels

LMC Truck Part Numbers:
- Cab Corners: 34-1473 (Left), 34-1474 (Right)
- Rocker Panels: 34-1469 (Left), 34-1470 (Right)
- Bed Sides: 34-2473 (Short bed left), 34-2474 (Short bed right)
- Tailgate: 34-2477 (Complete assembly)
- Front Fenders: 34-1432 (Left), 34-1433 (Right)
- Hood: 34-1437 (1973-1980), 34-1438 (1981-1987)

Fasteners and Hardware

  • Body mount bolts: GM part 14056533 (M12 x 1.75 x 120mm)
  • Fender bolts: Various lengths, typically 1/4"-20 and 5/16"-18
  • Door hinge pins: GM part 20405041
  • Tailgate hardware kit: LMC 34-2479

Paint Materials

Paint Systems

Single-Stage Restoration (Period Correct)
- Primer: PPG K36 Epoxy Primer
- Paint: PPG Deltron DBC single-stage acrylic enamel
- Reducer: PPG DT870 (medium temp)
- Hardener: PPG DCU2002

Modern Base Coat/Clear Coat
- Primer: SEM 39694 High-Build Primer Surfacer
- Base coat: PPG Envirobase High Performance
- Clear coat: PPG 2021 Glamour Clear
- Activator: PPG 2020 Glamour Activator

Color Codes (Common Factory Colors)

1973-1980:
- Classic White: Code 10, PPG 9075
- Medium Blue Metallic: Code 24, PPG 4930
- Carmine Red: Code 75, PPG 70175

1981-1987:
- Summit White: Code 40, PPG 9340
- Dark Blue Metallic: Code 23, PPG 4923
- Victory Red: Code 81, PPG 70381

Tools and Equipment

Essential Tools

  • DA Sander: 6" pneumatic dual-action sander
  • Sandpaper: 80, 180, 320, 600, 800, 1000, 1500 grit
  • Spray gun: HVLP system (DeVilbiss FLG-5 recommended)
  • Compressor: Minimum 60-gallon, 5HP for continuous operation
  • Body hammers: General purpose and finishing hammers
  • Dollies: Heel dolly, general purpose dolly
  • Metal cutting tools: Plasma cutter or cut-off wheels

Specialty Items

  • Rust converter: Corroseal Water-Based Rust Converter
  • Body filler: Evercoat Rage Premium Body Filler
  • Primer surfacer: High-build for imperfection filling
  • Tack cloths: Lint-free for final cleaning
  • Masking materials: 3M Fine Line tape, plastic sheeting

Step-by-Step Guide

Phase 1: Assessment and Disassembly

Initial Evaluation

  1. Document current condition with photographs from all angles
  2. Identify rust areas using a magnet to detect body filler
  3. Check panel alignment before disassembly for reference
  4. Catalog hardware as components are removed

Disassembly Process

  1. Remove trim pieces: Chrome bumpers, grille, badges
    - Front bumper: Four bolts, typically 9/16" heads
    - Rear bumper: Six bolts on step-side, eight on fleet-side
  2. Remove doors: Support door weight, drive out hinge pins
  3. Remove hood: Two-person job, mark hinge positions first
  4. Remove bed: Eight body mount bolts, disconnect wiring harness

Phase 2: Metalwork and Rust Repair

Rust Assessment and Cutting

  1. Cut out severely rusted areas 2" beyond visible rust
  2. Test remaining metal with screwdriver probe for hidden rust
  3. Clean cut edges with 80-grit sandpaper or grinder

Patch Panel Installation

  1. Create cardboard templates for complex shapes
  2. Cut replacement metal with 1/4" overlap for welding
  3. Tack weld in place checking fit before final welding
  4. Complete welding in 1-2" sections to prevent warpage
  5. Grind welds flush using 80-grit flap disc

Body Filler Application

  1. Clean metal to bare surface with 80-grit sandpaper
  2. Mix body filler per manufacturer's instructions (2-3% hardener)
  3. Apply in thin coats building up gradually
  4. Sand with 80-grit when fully cured (20-30 minutes)
  5. Progress through grits: 80 → 180 → 320

Phase 3: Surface Preparation

Sanding Process

  1. Strip to bare metal using 80-grit DA sander
  2. Address imperfections with spot putty or additional filler
  3. Sand with 180-grit for primer adhesion
  4. Clean thoroughly with wax and grease remover
  5. Final wipe with tack cloth

Primer Application

  1. Mask adjacent areas carefully
  2. Apply epoxy primer in 2-3 medium wet coats
  3. Allow proper flash time between coats (15-20 minutes)
  4. Wet sand with 400-grit when fully cured (24 hours)
  5. Apply primer surfacer if additional build is needed

Phase 4: Paint Application

Single-Stage Process

  1. Final surface prep with 600-grit wet sanding
  2. Tack off surface immediately before painting
  3. Apply base coats: 2-3 medium wet coats, 15-minute flash time
  4. Apply final coat: Full wet coat for gloss development
  5. Allow to cure 24-48 hours before handling

Base Coat/Clear Coat Process

  1. Apply base coat: 2-3 medium coats for full coverage
  2. Flash between coats: 10-15 minutes at 70°F
  3. Apply clear coat: 2-3 full wet coats
  4. Final flash: 20-30 minutes before moving
  5. Color sand and polish after 7-day cure (if needed)

Year/Model Differences

Early Models (1973-1980)

Paint System

  • Factory finish: Single-stage acrylic lacquer
  • Primer: Lead-based primer (contains hazardous materials)
  • Color availability: More limited palette, earth tones dominant
  • Durability: Poor by modern standards, requires frequent maintenance

Body Characteristics

  • Thicker metal: 18-gauge steel in most panels
  • Simpler designs: Fewer body lines and character features
  • Chrome trim: More extensive use of chrome and stainless trim pieces

Late Models (1981-1987)

Paint System Changes

  • 1981-1984: Improved single-stage acrylic enamel
  • 1985-1987: Factory base coat/clear coat on premium colors
  • Primer: Switched to non-lead primer systems
  • Color options: Expanded palette including metallics and two-tones

Body Updates

  • Thinner metal: Cost-cutting measures reduced panel thickness
  • Design changes: More aerodynamic front end (1981+)
  • Plastic components: Increased use of plastic trim pieces

Model-Specific Considerations

K5 Blazer

  • Removable top: Special attention to weather sealing
  • Fiberglass components: Different repair techniques required
  • Limited color options: Fewer factory color combinations

Suburban

  • Extended body: More complex masking and paint logistics
  • Three-door vs. four-door: Body line differences affect painting sequence
  • Roof rack provisions: Factory holes require sealing if not used

Heavy Duty Models (C20/K20)

  • Commercial use: Often require more extensive metalwork
  • Different bumpers: Heavier-duty components affect disassembly
  • Fleet colors: Limited to basic colors in many cases

Tips & Best Practices

Professional Techniques

Surface Preparation

  • Never skip primer: Even over existing paint, use adhesion promoter
  • Control contamination: Work in clean environment, use air filtration
  • Temperature matters: Maintain 65-75°F during application
  • Humidity control: Keep relative humidity below 50% when possible

Paint Application

  • Gun setup: 2.0-2.2mm tip for primer, 1.4-1.6mm for color coats
  • Spray pattern: 50% overlap, maintain consistent distance (8-10")
  • Tack coat first: Light mist coat prevents runs on vertical surfaces
  • Work systematically: Hood, roof, sides, then detail areas

Budget vs. Premium Approaches

Budget Restoration ($1,500-3,000)

  • Spot repair: Address only problem areas
  • Single-stage paint: More forgiving application, lower material cost
  • DIY bodywork: Learn basic metalwork and welding
  • Used parts: Source good used panels instead of reproduction

Premium Restoration ($5,000-15,000)

  • Complete strip: Remove all paint to bare metal
  • Professional bodywork: Complex metalwork and panel replacement
  • Base coat/clear coat: Superior durability and appearance
  • New reproduction parts: Ensures proper fit and finish

Common Mistakes to Avoid

Surface Preparation Errors

  • Insufficient cleaning: Wax, grease, or silicone contamination causes fisheyes
  • Wrong primer choice: Incompatible primers cause adhesion failure
  • Rushed timeline: Inadequate cure time between coats
  • Poor masking: Overspray cleanup is time-consuming and expensive

Application Problems

  • Wrong reducer: Temperature-inappropriate thinners cause texture issues
  • Contaminated air: Oil or water in air lines creates finish defects
  • Improper gun settings: Poor atomization causes orange peel texture
  • Environmental conditions: Wind, dust, or temperature extremes affect finish

Troubleshooting

Paint Defects and Solutions

Orange Peel Texture

Causes:
- Gun too far from surface (over 12")
- Air pressure too high (over 50 PSI at gun)
- Paint too thick or under-reduced

Solutions:
- Wet sand with 1000-grit, polish
- Adjust gun settings and re-spray problem areas
- Color sand and re-clear (base coat/clear coat systems)

Fisheyes

Causes:
- Silicone contamination on surface
- Contaminated air supply
- Wax or polish residue

Solutions:
- Strip affected area, clean with silicone remover
- Add fisheye eliminator to paint (emergency fix only)
- Replace air lines and filters

Poor Adhesion/Peeling

Causes:
- Inadequate surface preparation
- Incompatible primer/paint combination
- Moisture in substrate

Solutions:
- Strip to bare metal, start over
- Use adhesion promoter over existing finishes
- Allow proper cure time between coats

Color Matching Issues

Fading and Weathering

  • Test spray: Always spray test panels for color match
  • Blend panels: Fade new paint into adjacent panels
  • UV degradation: Expect some color shift on vehicles over 10 years old

Metallic Color Problems

  • Mottling: Caused by improper spray technique or reducer choice
  • Color shift: Metallics are sensitive to spray distance and overlap
  • Solution: Maintain consistent technique, use proper reducer for conditions

Bodywork Problems

Warpage from Welding

Prevention:
- Weld in short sections (1-2")
- Allow cooling between welds
- Use backing plates to distribute heat

Correction:
- Heat shrinking with torch and wet rags
- Mechanical straightening with hammers and dollies
- Body filler as last resort for minor imperfections

Panel Fit Issues

Common causes:
- Reproduction panels with poor tolerances
- Frame damage affecting mounting points
- Incorrect installation sequence

Solutions:
- Check frame alignment before panel installation
- Use adjustable mounting hardware where possible
- Plan installation sequence to maintain proper gaps

This article represents established practices within the Square Body truck community. Always follow proper safety procedures when working with automotive paints and chemicals. Consult local regulations regarding paint disposal and shop ventilation requirements.