Question on rust treatment Fiesta MK4 98

Hello, I reported about 4 weeks ago because of rust at the rear wheel box, typical disease, at the Fiesta^^ HAbe that in the summer yes times derusted abggeschilten and then taken so Kombispachtelmasse where harder and all was already there, which then can be completely dried, polished primed and varnished with paint and clear varnish. after that it looked quite neat. now half a year later the rust eats again through from the inside I guess once more strongly and the product bursts a yesterday I was with an old schoolmate who had the idea of welding etc. He thinks that it would be best to grind the product off again and to derust then mim welding device to weld and twist with a thick layer, then to prime the tinning cleanly, paint and clear paint over it, he could do this to me, and thought that this would be the cheapest alternative to dispel the rust again. what do you say? Do you think that is full of sine or should I use it as a tool? Then when we are already sweating, I would like to have a bad look not a complete bonnet extension but only on the headlights the apertures. We want to make the bad look ourselves and now I farge what is to be considered? that may not be too much because otherwise it could be annoying with the police or at the next Tüv visit. Where could you put it best without having to make a lot on the bonnet? I thought open the bonnet and Then from there the back points and bend the sheet in front of it. I am curious about your answers. Jens