All-Metal Truck Floor Rust Repair (1990 F150 Demo)

Video Watch Time = 10 minutes

I should have known better, but the wall-to-wall vinyl carpeting on the floor of my 1990 F150 had been holding moisture against the metal for years. I ripped the stuff off when I discovered it, but not before a hole had rusted through the floor. Watch how I use steel, my MIG welder and some autobody solder to fix this damage almost as good as new. If you’re tired of the drawbacks of catalyzed autobody filler (and there are many), this is the technique for you.

What Happened

Water became trapped under a wall-to-wall rubber mat
Moisture caused rust on the driver’s side floor
Several small pinholes developed
One larger hole required a full metal patch
Passenger side remained factory solid

The Repair Strategy

This is a 100% metal repair, not a body-filler patch.

The plan:

Grind away rust to full-thickness steel
Cut and shape a steel patch
MIG weld the patch into place
Grind welds flush
Finish the surface using traditional body solder

No Bondo. No synthetic filler.

Why Use Body Solder Instead of Bondo?

Solder is metal — not plastic filler
Stronger and more durable
Bonds chemically to steel
Traditional auto body method used before body filler was invented
Easier to shape than many people think

This is an old-school technique that still works beautifully.

Step-by-Step Overview

1. Remove Rust

Grind to solid metal
Eliminate feathered rust edges
Prepare clean steel for welding

2. Fabricate the Patch

Use sheet metal thick enough for strength
Shape with aviation snips
Test-fit carefully
Remove surrounding paint for weld contact

3. MIG Weld in Place

Spot weld around perimeter
Protect wiring and trim with heat shields
Grind welds flush

4. Apply Tinning Butter

Flux + solder particles
Chemically cleans surface
Prepares steel for solder bonding

5. Add Body Solder

Heat with propane torch
Build surface slightly high
Grind and shape smooth
Add more solder if needed

6. Prime and Paint

Sand smooth
Prime
Apply topcoat
Repair becomes nearly invisible

Tools Used

Angle grinder
MIG welder
Aviation snips
Propane torch
Body solder
Tinning butter
Wire brush
Random orbit sander

Key Lessons

Trapped moisture under rubber mats causes hidden rust
Oil spray protection works — except where water is trapped
Full-metal repair is stronger and longer lasting than filler
Body solder is still a viable modern technique
Careful prep makes all the difference

FAQ Section

Can you fix a rust hole in a truck floor without Bondo?

Yes. Cut out the rust, weld in new steel, and finish with body solder for an all-metal repair.

Is body solder better than body filler?

Much better. Body solder is stronger and bonds chemically to steel. It’s more durable but requires more skill. And unlike body filler, solder won’t encourage re-rusting.