The image below shows a folded inner section being used simply to provide strength. The issue with placing one sheet or layer of steel on top of another is that moisture harbours between the layers and over time corrosion sets in.
Shot blasting to remove rust, corrosion, scale and paint.
Flaw: Shot blasting cannot get between the steel layers and overlaps, so these areas remain full or rust and corrosion.
Acid dipped to clean & treat the steel in preparation for galvanising.
Flaw: The acid does not breakdown or remove any remaining paint.
The bulkhead is then submerged in molten zinc galvanise.
Flaw: Remaining paint burns to ash due to the heat of the molten zinc galvanise. This ash (burnt paint) no longer protects the steel beneath but instead harbours moisture, leading to corrosion over time. Galvanise only bonds to clean steel, not ash.
Flaw: Due to the original bulkhead sections being spot welded into place and made from only 1mm sheet steel, the entire bulkhead is prone to distortion due to the rapid heating up and cooling down during the galvanising process.