welding & fixing rust

Peace,

i need some more descriptive analysis about rust. the best way to fix it. its by the fender wells. and on the drivers side it has rust under the door floorboard on the bottom near the sheetmetal. where the chrome goes . down there. and need to know the best way to fix it. i heard when you weld metal the heat changes the chemical configuration of the heated metal and causes the weld itself to rust in a couple years enstead of a generation or two!

later

clean the metal well,turn your machine down so it dont wash away your bass metal (practice ) maybe you wouldnt mind thicker metal on what you replace use (70 s2)wire and a tiger disk to clean ,on the side that doesnt getpaint to help prevent rust use cold galv,and dont be selfish with it