Windscreen Pillar Rust

Hello Everyone

Over the last month or two Ive been working through a list of jobs that need to be done in order to get my Golf in a roadworthy condition.
I'm finally up to the last one!



By rhyder, shot with Samsung S85 at 2009-11-20


I have rust holes on both of the front windscreen pillars, And am wondering what the best way to treat them will be.
I cant just fill them in, because for the road worthy certificate (Australian MOT equivalent) It has to be a structural repair
I have access to a MIG welder
I have opened the holes out to about 1.5cm in diameter using a nibler, but the metal around the edge of the hole is 0.6mm thick, compared to the original 1mm
I have taken out the windscreen and the rubber seal
I have reasonably good access to inside the pillar from under the bonnet


So to sum up, my questions are:

How thick does the metal need to be so that I can weld to it?
What is the best method to patch the hole back up?
           (my current idea is, after treating the rust, to slip a piece of metal behind the hole, and weld it in to place. From there I can body fill)

you clould plate it like you say, or you could turn the mig weld down low and tack it up carefully then grind it down n fill it.

chop out a put new in

that what i did in exactly the same place

What thickness metal did you weld in underneath?