The rear wheel wells are one of the worst spots for rust.
This is my parts car...
View attachment 297514
It helps to cake grease on that lip/ledge around the wheel well. It's messy but it helps. That lip always fills up with dust and dirt then when it gets wet, it stays wet for days, maybe weeks.
Even if it's not salt water, it's not good.
Oh well. such is life for our cars!