FWIW, my experience leads me away from dipping a spot welded body. Especially one with small box sections.
I have attached a pic of the front leg from a Lancia Fulvia Sport Zagato. For reference, this leg supports the front subframe (which contains the engine, gearbox, suspension and steering gear). The car originally spent a decade in the UK, got boiled in a container in Jo'burg and was put under a tree for a decade. Not good for an inadequate protected Bulganian steel.
Shortly after buying the car, I had the body dipped which revealed substantial structural and body rust and poor accident repairs.
(What is left of the leg is exactly as it looked after the dip)
After removing this leg, I retreated the surface rust then coated the leg in Kephos. Now, some six years after the dipping, which included 'rinsing and neutralising' the chemicals and body, rust can be seen leaching out of the seams. As you can see, it would have been reasonable to expect that this leg would be easy to neutralise given the size of rust holes!
I also started to rebuild the rusted out gutters of the hatchback. Zagato's design for these gutters included two tiny overlapping box sections. Given the closed ends of the boxes, it would have been almost impossible to properly rinse these boxes. To my horror when I started to unpick the spot welds of the good parts of the gutter, I found heavy amounts of rust still munching on the steel.
So, in summary, if rust can be seen leaching out of a seam which is accessible from both sides some 6 years after the dipping and neutraling, I'd be concerned what happens in the long term in the more difficult to get to box sections even if some drain holes are provided.
(And I understand any comment that questions the quality of the rinse and neutralising process used by the dipper. I have the same question)
Hope this helps in making an informed decision. Regards, Mick
Fulvia 1600HF, Fulvia Sport Zagato (project), Giulia Spider Veloce (project)
Last edited by OMWattsy; 02-14-2017 at 03:45 AM.