you clearly have no fucking clue what you are talking about.
How long is a patch going to last?
that depends on how well the patch is performed, if all you do is cut the patch weld it in, then prime and paint , not long at all. but if the patch metal is cleaned, de-scaled, wiped down and acid etched, before the paint process then it will last as long as the car runs.
If rust came through on a fender in one spot the whole fender is the same age so there is rust there it just hasnt come through yet.
NO…just fucking no. metal doesnt oxidize because its old, it oxidizes and corrodes when the bare metal is directly exposed to oxygen or any corrosive elements (such as salt, or water).
heat from welding speeds up the process of rust.
then explain to me how hot rolled steel doesnt rust faster than cold rolled?..its not the heat, when the metal is molten or extremely hot and impurities such as air come into contact with it the contaminate it and create a starting point for rust…why do you think there is pre-flow and post-flow on mig and tig welders?
i had a rust spot on my vehicle that bubbled through the paint and i did the repair right using nothing but touch up paint…4 years ago and to this day there is still zero rust in that area.
next time you try to argue about metallurgy because a couple of your buddies are fucking hacks and you think you know everything, open a fucking book first.