Replace Rusty Floor Panels
There are several ways to deal with a rusty floor. If the hole isn't too big, you can do a patch repair. For a larger repair, the damaged section can be replaced with a partial floor panel. If the rust damage is extensive, replacing the entire floor pan with a one-piece panel is the best way.
The method you choose is not only dependant on your skill and budget, but also what's available for your year, make, and model car. Years ago, there wasn't the choice of replacement body parts that there is today.
Back in the day, we often made patches for floorboards simply because new panels weren't available. If it's just a small area that needs repair, a patch panel is a valid option.
Read: Patch Panel Rust Repair
Full Floor Pan vs Partial Floor Pan
Curiously, replacing the full floor pan is actually less work than doing two partial panels. Think about it; if you replace left and right side partial panels and leave the original tunnel intact, you're welding about ten more feet of sheet metal versus replacing the whole floor.
After the old floor is removed, a one-piece floor will need only slight trimming before being welded in. A partial panel requires a lot of trimming.
Installed correctly, a full floor replacement adds value to a restoration. It also looks better from underneath.
There are instances where using a partial panel is preferred. For instance, if only the driver's side floor is bad, and the rest of the floor is good.
A partial floor panel should be trimmed down to cover only the damaged section. Save as much good, original metal on the car as you can.
Floor Pan Removal
Once you've decided on full or partial panels, the floor can be removed accordingly.
Install Full Floor Panels
If you've decided to replace the whole floor, you'll be drilling out a lot (about 50) spot welds.
Removing Spot Welds
Cutting spot welds is not a quick or easy job. They should be cut out slowly, meaning low RPMs on your drill.
When I started out in body panel repair, I made the mistake of attaching my spot weld cutter to an air drill. All that did was quickly break the cutting teeth.
Shop: Spot Weld Cutter Tool (extra tips included)
TIP: When drilling out spot welds, spray a little cutting fluid or oil on the cutting tip to keep it cool. Another tip is to center-punch the spot weld before using the cutter. This prevents the cutter from "walking" off it's mark.
Butt Weld vs Spot Weld
Your car's original floor pans were spot-welded in because it was the most cost-effective (cheapest) way to do thousands of vehicles. Rather than duplicate the factory spot welds, many restorers prefer to run a complete bead around the new panel.
Tip: If you're spot-welding, pre-drill the holes in the new panel. Not only will this save you time during installation, it will be more comfortable standing at your work bench than sitting inside the car.
Butt Weld vs Lap Weld
Many restorers choose to lap weld floor pans because it's easier. However, butt welding looks better and leaves no overlaps, but is a long process.
Read: MIG Weld Sheet Metal
Install Partial Panels
If you're doing a partial floor panel, you'll be cutting around the repair to be done. Tape off the spot to be repaired with masking tape. Mark off about one inch around the rusted-through part.
Only cut away what you have to, keep as much original metal on the car as possible.
With an air-powered cut-off wheel, slice along the taped line, letting the air tool do the work. Don't force it to cut faster, as this will only overheat the tool and the metal.
Read: Best Air Tools For Automotive Restoration
After the old panel is removed, go over the edges of the good part of the floor. A 4-1/2" electric grinder with a wire wheel works well, or you can use an air-powered sander. You want to have a shiny, solid rust-free surface to weld onto.
To help protect the new sheet metal, spray both sides with a zinc-rich primer. These are offered by many different companies. The primer gets sprayed on after trimming but before welding.
Trim And Fit New Panel
After cleaning both mating surfaces, test fit the panel to the floor several times. Clamp them into place while aligning everything up.
Tack on a few welds to hold the panel in place. Then stop and put the welder down. Step back, check it, then check it again before final welding.
Alternatives To Welding
You may have heard that many new cars are being assembled with structural adhesive right from the factory. Yes, that's true, but they also go through an oven to cure the adhesive.
Epoxy/Adhesive and Pop Rivets
This is the low buck method that some of us used before learning how to weld. After getting the replacement panel or patch cut to size, rivet it down and apply seam sealer (usually from a tube). It' not pretty but it gets the job done. Please note that this type of patch will be very difficult to remove in the future.
After mixing up a two-part resin, the resin gets poured over a mesh screen, which covers the metal to be repaired. A squeegee or fiberglass roller is then used to work out the air bubbles. Unless you repair fiberglass panels for a living, I do not recommend this method.
Caution: On unibody cars, the floor is part of the body, which is part of the vehicle's structure.
Learning To Weld
If you're just getting started with automotive bodywork, a wire-feed (MIG) unit is a good choice for a welder. They work well with thin sheet metal, easy to learn on, and most people can produce quality, good looking welds with minimal practice.