Protecting the frame of your 1999–2006 GMT800 against rust is one of the best investments you can make. Here’s what works.
Why Undercoat the GMT800 Frame?
The factory frame protection is a light paint application that wears off over time. Road salt and moisture cause surface rust that progresses to pitting and eventually structural damage.
Top Undercoating Options:
Rubberized Undercoating Spray:
Rust-Oleum 2-in-1 Rust Reform & Seal Spray – For areas with existing rust, this converts and seals in one step before applying undercoating.
Professional-Grade Petroleum Undercoating:
Used by shops for annual applications. Penetrates existing rust and seals. Typical cost: $150–$300 for a professional frame spray.
Application Tips:
- Pressure wash the undercarriage thoroughly
- Allow to dry completely
- Treat any rust spots with converter first
- Apply undercoating in a 70–80°F environment
- Focus on frame rails, crossmembers, spring hangers
Best Practice:
Annual inspection + touch-up is more effective than one heavy application. Check for areas where previous coating has flaked or been chipped by road debris.
Internal Frame Treatment:
Use cavity wax or internal frame coating sprayed inside the boxed frame rails to protect from inside-out rust.
What undercoating are you using on your GMT800 frame?