Are you in danger of putting your feet through the floor, Fred Flintstone-style, in your streeter? After sills and doors, the most common area of rust in older cars is the floor pan. Vinyl mats with the fibre underlay are the biggest culprits. Moisture works its way in via wet feet and windscreen leaks, which the underlay retains. The vinyl mats cause sweating which inevitably leads to corrosion and before long you’re in danger of joining the Flintstone clan!

First published in the December 2004 issue of Street Machine

You could just patch up small areas but the floor makes up an integral part of your car’s chassis (all cars without a separate chassis use the vehicle’s monocoque as the chassis) and it needs to be structurally sound. Therefore it’s best to replace complete sections, fixing like with like. You could take rust-free sections from a donor car but finding a pristine donor is a tedious task and then you’ll need to laboriously unpick the section without destroying it.

A more user-friendly approach is to use an aftermarket rust-repair panel from one of the many suppliers such as Rare Spares. These sections already have the OEM pressings needed to fit your car and are a pretty close match to the originals. Best of all, they’re stamped from fresh, rust-free steel. Watch on while Streetneat Panels runs through a typical repair.

STEP-BY-STEP

1. It’s not uncommon for cars which are otherwise pristine and sound to suffer serious corrosion like this in their floor pans. Sandblasting highlights the areas needing attention. It’s best that areas to be repaired are as clean as possible and stripped free of anything flammable (grease/tar/underlay etc).

2. The Rare Spares pans are quite generous, therefore it’s often desirable to trim away excess rather than install the entire section.

Once trimmed, lay it over the existing floor and mark around the affected area. In extreme cases you may need to add a section of floor from a donor vehicle to fill in any gaps.

3. All spot welds where the old floor is attached to the underfloor members must first be drilled out. Tackling this job from underneath makes the spot welds a lot easier to find, as does having the car sandblasted first! Stitch welded areas can be trimmed with an angle grinder or an oxy torch.

4. Using a four-inch grinder with a thin 1.0mm-thick cut-off disc, the bulk of rusted floor can now be trimmed away — make sure you keep within the marked chalk line by at least 10mm. This allows for adjustment and heat shrinkage caused later on when the new pan is welded in place.

5. Often, simply drilling out the spot-weld is not enough. Usually you’ll need to hammer a screwdriver, cold chisel or heavy-duty paint scraper (as used in this case) into the joint to break the last bit of some of the spot welds. A brickie’s scutch (or swedge) can used if you need something a bit more heavy duty.

6. Wrestling out the old pan will soon tell you if you’ve successfully broken all the welds. Sometimes it’s best to leave some of the small, inaccessible areas attached and clean these off with the grinder once the main section is out of the way. This way you’ve got better access to all the fiddly areas.

7. With the rusted floor section removed it quickly becomes apparent how much rust and crud can be accumulated in the sills and frame rails. Damaged or corroded underlying chassis members and drainage points may need to be repaired or even replaced in severe cases before continuing with the repair.

8. After cleaning up the welds and rust with a grinder or wire wheel, paint all the areas which will be inaccessible later on with an etch primer or a similar sealing oxide primer — be thorough here otherwise the rust will quickly return.

9. Adjusting the new repro pan with some careful persuasion is going to be necessary. A brickie’s swedge is used to make the edges line up and forces the pan into position against the side braces.

Take your time and be careful not to overdo it otherwise the new pan can end up losing its shape!

10. Even with deft persuasion the new pan will not sit exactly in position by itself. Therefore it’s not a bad idea to have a selection of Vice-Grips on hand to firmly hold the new pan section in the desired position while hammering and welding.

11. With the new floor pan trimmed and fitted, re-weld all the spot welds (plug weld) using the MIG. Once again, this is a job best tackled from underneath, welding through the holes left behind from drilling out the old spot welds. An assistant pushing firmly from the top keeps the pan snugly in place.

12. After plug welding the new pan to the original members and sills, the rest of the floor section can be welded in place. Although MIG welding is perfectly fine, Streetneat prefers to oxy hammer-weld any repair section, as it gives the joins a neater finish. Here Kim holds the pan down while I start welding.

13. Overlapped edges trap moisture so I prefer to butt-weld all the edges. To achieve a perfect butt, weld a small section then run the four-inch angle grinder along just in front to continuously trim away the excess. The thin cut-off wheel leaves a perfect 1mm gap for welding, requiring minimal filler wire.

14. Kim provides the other half of the oxy hammer-welding process (from underneath the car). I weld a small section and while it’s still red-hot Kim holds a dolly up to the weld as I hammer it flat. Alternatively, when the floor starts to suck in the wrong direction Kim can hammer it back up while I hold it from the top

15. Once fully welded it should look like this. Using the combined efforts of two people makes for a neater repair. With the pan plug-welded, it’s not going to shift.

However, work on one edge at a time in small sections (especially if you’re MIG welding) to keep warping (due to excessive heat) to a minimum.

16. Use a four-inch angle grinder to dress all the welds. How good a welder you are will determine how much clean-up grinding you’ll need to do. Also, how far to go with disguising the repair is up to you.

Take your time and be careful not to get carried away, otherwise you could end up grinding out all the fresh metal you’ve just put in.

17. For that show-winning finish, a small wipe of filler underneath will make it look like it never happened. Ensure all the joins are wiped with seam sealer top and bottom and the whole lot given a coat of etch primer then primer, otherwise the dreaded tin worm will be back before you know it.

WRAP UP

Rare Spares manufactures a variety of rust repair sections for a wide range of vehicles. Replacing a generous area around any cancer-ridden metal is the best way to stop it recurring. As for whether you get a show-winning finish as opposed to a functional repair, that’s up to you — patience is the key.

Application of spray-on Stone Guard or Body Shultz tar will blend the repair into its surroundings.For this type of repair (to one floor pan section), expect a shop to charge approximately one day’s labour plus materials. You could probably find a shop to do it for around cheaper if you’re just after an overlapped, MIG welded affair to keep your daily runner legal.

While butt-welding is preferred (it gives a neater, more factory look) you can get away with overlapping the two panels if you’re MIG welding the repair. However beware! Overlapped panels, if not properly sealed, will cause rust later on down the track.