What We Were Dealing With
The bus came from a rural Michigan school district. Michigan roads are heavily salted — which means school buses from the Midwest and Northeast are far more likely to have serious floor rust than buses from dry states like Arizona or New Mexico. The listing said "some surface rust." It was not just surface rust.
When we pulled the rubber floor mats and plywood subfloor panels (most school buses have a plywood layer over the structural steel floor), we found about 40% of the structural steel floor had active rust, 15% had perforation (holes rusted through), and 3 crossmembers needed to be professionally welded in. Total unplanned cost: $680 in steel and welding.
The Complete Rust Removal Process
Step 1: Remove Everything
All seats, floor mats, loose plywood, and any remaining wheel well covers. Get down to bare steel. Use a heat gun to soften stubborn adhesive under old mats. Budget a full weekend just for demo — it's slower than you think.
Step 2: Assess and Mark
Use chalk to mark every rusted area. Categories: surface rust (orange chalk), deep pitting (yellow), perforation (red). Red zones need welding or steel plate patching before anything else happens. Call a local mobile welder for the serious stuff — it's usually $60–$100/hr and worth every penny.
Step 3: Mechanical Removal
Angle grinder with a wire cup wheel for heavily rusted areas. Flap disc (80 grit) for moderate rust. DA sander (80 grit) for surface rust on flat sections. Goal: bright bare metal wherever possible. You will go through many wire wheels. Buy a 10-pack. Wear a respirator — rust dust is not good to breathe.
Step 4: Chemical Treatment (Naval Jelly)
Apply Naval Jelly or a phosphoric acid rust converter to any areas you couldn't fully grind to bare metal — tight corners, under flanges, around bolt holes. Let it work for 15–20 minutes until the rust turns black/dark purple. Wipe off and let dry completely. Do NOT apply POR-15 over wet Naval Jelly or you'll get adhesion failure.
Step 5: POR-15 — The Important Rules
POR-15 is one of the best rust encapsulators available, but it has specific rules that will ruin your job if you ignore them: (1) Apply in low humidity conditions — above 60% RH causes blushing and adhesion failure. (2) Don't let it dry in sunlight — it cures too fast and becomes brittle. (3) Apply two thin coats, not one thick one. (4) It MUST be topcoated within 96 hours or it becomes too hard to bond with anything.
Step 6: Topcoat & Seal
We used POR-15 Top Coat in chassis black over the POR-15 base. Two coats. This protects the UV-sensitive POR-15 underneath and gives a clean finished surface. The result is a floor that looks factory-done and is protected against future rust for years.
Step 7: New Subfloor
Once the metal is treated and fully cured (48 hours minimum), lay new subfloor. We used 3/4" treated plywood on the structural floor, then 1.5" XPS foam insulation, then a second layer of 3/4" birch plywood as the finished subfloor surface. This gives R-7.5 floor insulation and a solid substrate for the finished flooring.
Products Used
Ospho (Phosphoric Acid Rust Converter)
Better than Naval Jelly for heavy rust — penetrates deeper and leaves a harder primer surface.
POR-15 Rust Preventive Coating
The industry standard for rust encapsulation. Extremely durable when applied correctly.
POR-15 Top Coat
UV-stable topcoat required over POR-15. Don't skip this step — bare POR-15 degrades in sunlight.
Total Cost of Our Floor Restoration
| Item | Cost |
|---|---|
| Welding (3 crossmembers + 4 patch plates) | $680 |
| Wire wheels, flap discs, sandpaper | $68 |
| Ospho (2 quarts) | $44 |
| POR-15 (2 quarts) | $70 |
| POR-15 Top Coat (1 quart) | $28 |
| Treated plywood subfloor (6 sheets) | $186 |
| XPS foam insulation (12 sheets) | $108 |
| Birch plywood finish subfloor (5 sheets) | $155 |
| Screws, adhesive, misc | $42 |
| Total Floor Restoration | $1,381 |