galvanised?

well, galvanising of body parts is usually done at manufacture. Typically, these days, zinc-coated steel sheet is used. Or the parts are hot dipped after pressing and cutting. Sometimes, however, this is only on the inside of a panel, since the galvanising reduces the appearance of the final painting. According to recent brochures the G is supposed to be galvanised, as always.

Other than the use of zinc primers, I don't think that this is something that can be done after manufacture.

Mercedes have used / use Bonazinc coated steels . This is a zinc-loaded expoxy primer. applied to the metal sheet by the steel manufacturers.

for example:

http://ds.arcelormittal.com/benelux/mosacier/14014/language/EN

Problem is that where the steel is cut, the edges are not then coated and that is where corrosion can start.

In the late 90's, early 2000's MB had serious rust issues on many cars through

• the use of this (single-sided) zinc-pre-coated steel, with uncoated edges.
• new, water-based primers which did not adhere properly to the metal, allowing moisture to track in from the edges, such as door bottoms or wheel arch edges.
• bacteria which developed in the primer vats and which further reduced adhesion
• inadequate paint thickness
• abandonment of sealing of panel seams (eg around doors)
• abandonment of  underseal coatings

1990's Gs appear to be painted with traditional paints, which are tough and durable. I don't know whether Graz switched to water-based paints around 2000 ?

However, water-based paints are not a problem in themselves, but as with the introduction of any new process or technology, there will be teething troubles. Post 2003, MB addressed some of the problems, by switching to fully-galvanised panels, and sorting the issues with the adhesion of the water-based primers.

This weekend I was trying to address some small rust issues on the bottom edge of a door on a 2001, S-Class. I used some low-tack, blue masking tape to mask off just the bottom edge and was surprised when I removed the tape -minutes later - to find that the paint and primer had come-off with it, revealing shiny metal beneath. This was a patch the size of a 50p coin, mid panel, not peeled  from the edge. Obviously, poor or no adhesion of the primer to the metal.

The total  paint thickness was also incredibly thin. No wonder there are so many rusty mercs around from that era.

What you describe is quite common on a W460. If you need to replace panels, then I would buy them from an MB dealer parts dept, not pattern-parts off the internet. You can then coat them with a zinc-rich, weldable primer  and replace the parts. Where your have access, you can follow-up with re-painting from the inside, but in all cases, use a good, penetrating wax-sealant to finish the job.(Dinitrol ML3125). The sill should have access holes for cavity wax injection. Likewise the internal door cavities should also be treated with wax, and pay attention to where the door-seal fits and the metal edges, there.

It's likely that the replacement panels will already be galvanised, or manufactured from coated steel, but if not and If you wish to go to the effort, you can take the new panels to be stripped and galvanised, but it will still be important to properly seal from the inside.

Anyway, galvanised panels will help, but ultimately, they will not prevent rust when exposed to mud and road salt. The zinc coatings are relatively thin and its like the plating on nuts and bolts, which eventually rusts through.

I think the best approach  is a good coating of quality paint and regular cleaning and 'waxoyling'

if you presently only have surface rust, then that can be treated and you can probably save your existing panels.