Repairing rusty floorboards – ’88 F450

Several weeks ago, I pulled my aging F450 Super Duty rollback into the shop to fix a few things as the annual inspection was due.    Nothing major – just a few marker lights on the bed,  replacing the flood lights on the rear of the truck, and pulling the steering wheel to address the turn signal mechanism.  It worked fine for right turns, but signaling left required you to fiddle with the lever a bit before the light would blink. It had really been that way since I bought the truck in 1996, and was an annoyance, but something I’d never made time to address.

Having fixed these items, I noted some surface rust in the door sill areas and inside bottom edges of the doors.   I knew the molding was loose on the driver side and some minor rust had been there as long as I’ve had the truck.  It was on my list to fix, along with the vinyl floor that was stained, torn and pretty ragged – more things that just hadn’t been a real priority.   Peeling back the  floor covering turned out to be  like pulling a loose thread on a sweater – things unraveled quickly.  The vinyl ripped, the padding underneath was a damp, oil stained, moldy, rotted mess.  Worse, there was rust, rust, and more rust.   So,  stripped everything out and dove in.

Both the driver and passenger foot wells were a rusty mess – there were holes and lots of rust and scale.   I attacked these with a knotted wire wheel in a grinder to strip down the metal and used a shop vac to clean up the resulting mess.  A dust mask is highly recommended.

The wire wheel removed the surface rust and the holes seemed to multiple and grow bigger and some of the remaining metal was pretty thin.    Welding in patches was going to be a challenge.

Perhaps the “right” way to address all this would be to order reproduction floor panels and cut out a lot of material and weld in entire new sections.

A backyard fix would have been to use fiberglass mat and resin to rebuild it, or to just cut up some thin sheet metal with snips and either pop rivet or screw it in place.

Having access to a lot of 14 gauge scrap steel (which is about twice as thick as the original material), I decided to weld in patches.

I started with some of the smaller holes and cut patches that covered the holes with a lot of overlap, and adjusted my mig welder to a very low amp setting.  A bit of fine tuning with the wire feed speed and I was able to weld without burning through too much.    Some welders have a stitch mode, where weld time can be set – I experimented a bit and found this helped reduce burn through in areas were the original steel was pretty thin.

I drilled holes in the one patch piece and made plug welds to tie into the cab floor reinforcement channel under the floor.

I moved on to address  the larger areas once I had the small spots sorted out and the welder adjusted so that I wasn’t just making more swiss cheese out of things.  Here was how the driver’s side looked as I began work.

First, I cut long strips of plate and began by rebuilding the edge of the floor along the doors.

The welds aren’t pretty, but they are solid – even welding in very short pulses – just long enough to develop a puddle, it was easy to burn through, and then I’d keep working back until I could close the holes.

With a solid and proper edge to work from, I began to add plates to join the floor pan to the edge of the door frame.  Since the factory had stamped irregular shaped depressions into the floor, I had to add small pieces to fill these in, so that I could then bridge across them with larger pieces.  I felt this was a better way to ensure there would be no open sections or gaps from below.

I also rebuilt the vertical flange that runs the width of the door frame and holds the weather stripping in place.

After all the welding,  I used the wire wheel to clean everything again, then sprayed the floor from above and below with a rust neutralizing agent (that turns it black) and then filled and sealed it with bondo, followed by two coats of primer and then a coat of red.

After everything dried, I installed new fiber padding, and a new black vinyl floor which, while advertised to be pre-molded, required a bit of work with a razor knife and heat gun to install.   The heat gun proved to be a real aid in forming things to shape, stretching around the floor shift, etc.

I located the seat and seat mounting holes by pressing a screw driver up through the mounting holes and then trimming a dime sized clearance hole around them with a razor knife.

The seat then installed easily, along with the weather stripping and trim plates.   The finished job looks pretty good.

Of course,   I’ll  now want to have the cab repainted in the spring.