The floor panel--commonly known as the floor pan--is a common location for rust on a Mustang. There are two ways to repair rust on the floor pan. If the rust is minimal and positioned in one central location then patching the floor pan is the best option. If rust exists in many locations or the pan is completely rusted, replacing the whole floor pan is necessary.
Lift the carpet off the floor of the Mustang. To do so, remove the doorsill plate. If the sill plate is metal, remove the screws that secure it to the floor. If the sill is plastic, pull upward on it starting at the end closest to the rear passenger seat until it comes loose. Locate the front edge of the carpet under the dashboard and pull it backwards. Roll it towards the front seat and duct-tape it to prevent it from unrolling.
Place the replacement floor pan on top of the original pan and trace an outline of the replacement floor pan on top of the original floor pan with a marker. Set the replacement floor pan off to the side for now.
Draw a second line around the tracing on the original floor pan approximately 1 inch from the outer edges.
Cut the inside line with a die grinder or cutting wheel to remove the original floor pan from the Mustang. File the rough edges of the hole with a bastard file.
Place the new floor pan in the Mustang and check the fit. The edges must lie flat against the remaining portions of the floor pan. If the edges do not fit, remove more of it until the new pan fits properly.
Drill holes approximately every 2 inches around the edge of the new floor pan. The new floor pan should overlap the existing pan by 1 inch. Drill the holes inward from the pan's edge--approximately 1/2 inch. The size of the drill bit is dependent on the size of rivet you use. The box the rivets come in will indicate the proper drill bit size.
Insert rivets into the holes with a rivet gun.
Place a bead of RTV silicone sealer around the edge of the new floor pan to seal the new and old floor pan. Or you can weld the new and old floor pan together instead of using silicone.
Grind away the rust with a die grinder or cut the rust spot away with a cutting wheel. Remove all of the rust before proceeding. If any rust is left on the existing floor pan, it will continue to spread.
Cut a square piece of sheet metal large enough to cover the cut-out spot by at least 2 inches. You can cut sheet metal with hand-held metal shears, a band saw or a circular saw.
Place the piece of sheet metal on the floor pan so that the edges sit flat against the existing floor pan. Striking the piece of metal with a rubber mallet usually works to flatten it.
Drill holes approximately 1/2 inch from the edge of the sheet metal patch for the rivets. Check the rivet box to see what size drill bit you should use.
Rivet the patch into place with a rivet gun.
Seal the edge of the patch with RTV silicone sealer or weld the patch into place.
