The rust was quite extensive in this car. I've cut off the sills, the lower suspension mounting, and the side and bottom of the chassis. The metal that's left is in good condition.
I used a metal cutting disc on an angle grinder to roughly remove the rusty metal, then used a grinding disc for removing the rusty parts from flanges. I try to maintain the original construction as far as possible.
The shiny metal at the edges was prepared by lightly grinding off the rust and paint. It's really important to weld to clean, shiny metal - this is electric welding after all so there needs to be a good electrical contact between the torch and the metal you are welding.
