Ja, sorry to spoil your hope! Not saying that it cant be done. BUT, even with a 1.2mm rod, it will take a real (no seriously!) professional qualified welder to weld this metal! Bear in mind that you say that the metal is vrot, and even with a MIG welder, you are going to have a real kak time trying to weld your braces in. Rusted metal dont weld, it 'evaporates' into thin air as soon as the 'spark' of the MIG welder strikes...... Sorry.
What realy need t obe done is to determined EXACTLY where your rust stops, and then cut at least 1cm higher on the panel that needs repair, and work from there.... What you also have to bear in mind is that in 90% of the cases, you cant see the rust that is on the REAR of the panel where you want to do the repair job. Soooooo......, even when you think that you are working on non-rusted metal (looking from the outside), the panel does have rust on the backside..... After welding in your repair panel, there is still quite a bit of work left to do in treating the remaining rust that is present on the back. Otherwise you nicely new welded repair panel will start to rust in double time(remember, wherever you weld, you burn the existing paint off on the backside, and a lot of people dont think properly to treat this new fresh bear metal. And that is where your sh!t starts!

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Hope this input helps a little, sorry for the longwinded post.