Ill try and explain all yeah...
1. If the panel is SO rusted until theres a hole = Must replace the panel completely, take a piece of sheet metal and re-weld on. Usually this is very rare unless youre restoring some kind of classic car of some sort...
2. Spot welding. Imagine 1 piece of metal which is 3cm long, every 3cm they weld 2 pieces of metal together. So in between of the 3cm, that part can flex right? Spot welding welds in that middle part to make the 'flex area' smaller. Then after the spot welding the part that can flex is only 1.5cm etc. etc.
3. Bracing. Imagine your arm + a piece of metal bolted to it. Sure kuat right?
4. Chassis foaming. My own Nissan has been foamed, basically they just inject this foam thingy into the hollow sections of the cars chassis like the a-pillars (windscreen pillar) and so on. When it hardens it becomes like cement but light, so it strengthens it from the inside... Try imagine your hand being the car, and pretend your bones are hollow. The foam makes the INSIDE solid while the bracing makes the OUTSIDE stiff
And thats what chassis strengthening is! How you want to do it is totally up to you... :rolleyes: