Modification Modification
Car: Ford | Fiesta | CX (Mk3) | All Derivatives | Hatchback

Seam welding the bodyshell

The seams on the majority of production road car body shells are spot welded at approximately 3" intervals. For manufacturers this permits high speed, cost effective construction but, from a performance perspective this is far from ideal! The amount of flex in a 3 door Fiesta bodyshell becomes apparent when the car is raised on one side; the doors will be slightly out of alignment. The Mk3.5 offers much greater rigidity but it is still far from perfect.

Seam welding considerably strengthens a standard bodyshell; it is relatively simple but very time consuming process that involves intermittently welding all structural seams on the car. Intermittent welds are preferred as a continuous weld can warp the metal and also promote cracking in vital areas due to massive reduction in the stress absorbing ability of the shell. The traditional consensus was to weld an inch and leave an inch, but I have noticed a few chassis today are being prepared with 3/4" on and 3/4" off so this is what I have chosen

I am seam welding my shell for 3 reasons; 1) a 400% increase in power over the original car 2) much improved handling 3) safety.


The original interior

Here is the shell as it left the factory (the white sealer was my original attempt at clearing the typical Fiesta leaking bulkhead problem - it seems to have worked!).

Exposing the seams

The first step is removal of anything that is likely to melt during the welding process. I stripped down the car to the absolute bare shell, including all of the trim clips which are to be replaced with new items when the car is rebuilt.

The bulk of the work is removing the OEM sound deadening and seam sealer, for both of these I used a heat gun and scraper. Bitumen residue from the sound deadening is easily taken care of with petrol and old rags. Some of the seam sealer was quite awkward but nothing a blow torch didn't sort out. Finally I used paint stripper and a wire brush to expose the bare metal along the seams.

All in all this stage of the project is very tedious but worth doing thoroughly to ensure good quality welds.

I sprayed any external seams with a copper based weld-through primer called Metaflux. The theory is that this protects behind the weld and in the seam, in practice however I've no doubt that most of it burns off, although any extra protection however small is worth it in my eyes. There were also patches of bare metal that were exposed after scraping off the sound deadening and I covered these with acid etch primer.


I have welded all of the structural seams inside of the shell, including some non structural areas such as the steering column. The bulkhead will also be reinforced at a later date particularly around the steering rack and brake servo areas.

Boot area

Bulkhead area

Main floor pan



I primed the seams with etch primer on the same day as the welding took place.


Sealing the seams

Instead of leaving the seams open I wanted re-seal them, partly for neatness but mainly to prevent corrosion and seal the passenger compartment from the elements as much as possible.

I chose a brush on sealer for an OEM finish but did plan to improve on the Ford application by using masking tape to restrict the area around the seam to which the sealer was applied. In practice this turned out to be pretty useless as all it served to do was create a large ridge of sealer at the edge of the tape. It actually looked neater brushed on so I left it at that.

I finished all of the seams; even those on the roof line, just for continuity.

© Mark Stewart, Saturday 18th February 2006
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