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After last month's aborted repair job, Project 13/30 gets a new patch of floorpan and some paint.

(Words and Photos: Dave Smith, American Car Magazine, February 2016 issue)​

In last month's instalment, I'd tried to repair the spiderweb of cracks in the driver's side floorpan around the seat bolt. The floorpan is pitifully thin, and had stretched before it had cracked, so I was trying to weld sub-one-millimetre thick metal …it was never going to end well. I gave it a shot, but the results were utterly sub-standard.

I have the donor car in the yard, and while it was rot that took it off the road in the first place, I thought I'd check the floorpan. It didn't bode well when I shunted the driver's seat forwards and noticed that the carpet was wet through (1).

Looking on the bright side, though, if the pan was watertight that meant it hadn't rotted through! I removed the sill trim, driver's seat and lower seat belt mount, plus the bracket that holds the fuel pump relay (conveniently placed beneath the driver's seat, for reasons unknown) and lifted the carpet. Yep, I had a water feature (2), but lifting that oval aluminium floor pan plug drained it, and it seems the pan itself was pretty strong.floorpan cut

Well, it was until I cut a socking great hole in it (3).floorpan new piece

I cut the patch panel big enough to replace all the area that had cracked (4). It was also interesting to note that Ford had later attempted to address the issue of cracking floor pans with this little load-spreading plate spot welded onto the inside of the pan around the rear bolt holes (5).

I cut out a hole in the cracked floorpan to remove the existing seat bolt hole and make enough room for that load-spreading plate (6),

So I could offer the new piece up flush to the floorpan (7).

Yes, that covers the damaged area, so I cleaned the panel back to bare metal (8 ) and gave it a coat of weld-through primer (9).

I then held the new panel in place and drew around it with a Sharpie (10) and cut out the floorpan about 10mm inside the Sharpie line (11). I then used a joddler, or edge-stepping tool (12), to set a shallow recess into the floorpan (13), and to make an opposing recess on the panel I was about to weld in (14).

Having my lovely new Clarke 160TM MIG welder helped a great deal, but that didn't mean that I was suddenly an expert welder or that the metal was anything other than thin tin. Having turned the power and wire speed down to a point where I could make a good weld without blowing through the steel, I welded the new piece in from underneath, dressed the welds and gave it a coat of primer (15). The beauty of having used the edge-stepper is that I could then go inside the car and weld another seam, and although the results are far from pretty (16), they do seem to be strong.

In theory, at least, the floorpan doesn't need all its strength because the car is about to get a set of big, solid subframe connectors courtesy of BMR Suspension, seen here just resting in place (17). As you can see, the cross-piece also braces the floorpan via the seat bolts!

On early cars, the seats were held in by studs and nuts, while later cars used longer bolts - these subframe connectors require the longer bolts, attaching the cross-pieces with nuts, so I had to remove the existing studs using the old double-nut technique (18 ). Also, I was slightly concerned by the fact that the U-section of the connectors, where it wraps around the chassis leg under the driver's and passenger's feet, was slightly wider than the chassis leg itself (19). However, the subframe connector U-section is made out of four or five millimetre steel and the chassis leg is stamped out of, perhaps, one millimetre steel, so I was going to run into the same problems I had found with some of the UMI suspension bracing brackets I'd used at the rear - trying to weld thick steel to thin is unlikely to end happily.

Once again, I broke out the Sharpie marker and marked where the subframe connectors would attach (20). I cut out four strips of 2mm steel (21) and cleaned them up, thinking that I could sandwich these between the connectors and the chassis rail. They should weld easily to the chassis rail, and take a bit more grunt from the welder when it comes to fitting the connectors. They also helped reduce the gap at the bracket (22), though there's still enough room for another sandwich piece, so I may have another on standby when it comes to fitting, then see which side it needs to go on.

I cleaned up the area where the strips need to go (23) and held the plates in place with a G-clamp (24). I then seam-welded the upper three sides of the plate and tacked the fourth. While I was at it, I also tacked the very ends of the gearbox crossmember brackets which were poking up, then dosed the lot in primer (25). While that was drying, I went all around the underside sanding down the rough edges on many of the spot-welds (26) - cutting your fingers on poorly finished factory welds is a novelty that wears off very quickly.

I went around the inside of the car, masking off all the holes (27) then scuffed the underside with a sanding block and blew it down with the air line because it's time for paint. The paint I'll be using is an Eastwood Extreme Chassis Black via Frost Auto Restoration Equipment, specifically designed for maximum adhesion and protection for chassis and underside (28 ).

It comes as a specific primer, then a topcoat, and both are thinned to sprayable consistency right from the tin. There's a lot of solids in there, and the instructions say to stir for at least a minute prior to painting - good old coffee stirrers (29); if these coffee shops are going to charge me £3 for a brew, they can't complain when I nick a handful of stirring sticks each visit! The paint also says that the ideal temperature should be between 21 and 30 degrees centigrade. Best of luck with that...

The primer went on beautifully (30), and made even my ham-fisted spraying look good. It says to give it two coats with 24 hours in between, which is what I did.

To cover the complete underside twice took me two quart cans, and left me with a tiny bit spare for touching in. When the second coat dried, it looked absolutely lovely (31) so I immediately thought I'd better go and ruin it.

Seam sealer is horrible, mucky, sticky, rubbery stuff, but if you want to keep your rust-free car rust-free, it's a necessary evil. Brush-on seam sealer is a simple enough concept, but it's far too thick for the average brush. I got a couple of normal paint brushes and trimmed the bristles down by about half (32). Supposedly, one swipe with the brush should do the trick but there are dozens of seams and joins under the Fox, and I wanted to make sure each one was sealed - not only to keep water out, but also dirt, draughts and noise - so I worked the sealer right into every gap (33). You have to watch out for drain holes - if a box section is open at the top, there may only be a narrow gap at the bottom to drain it, and if you seal that hole, you've created a whole new problem for yourself...

In the end, I used almost the whole 1 kg tin of seam sealer to cover every nook and cranny underneath the car (34). I don't want this thing to ever see rust, so while it's not pretty under there, it should be water-tight. Next comes the chassis black topcoat, which will hopefully leave it looking presentable, so stay tuned!
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