I haven't read anything above me but I will tell you what I did after I welded in my new floors. POR15 with no prep lol seriously though. Brushed off any loose dirt and just brushed onto to straight bare metal. As for the driver side which is the same situation as you. I simply cleaned off the grease, let it dry, Prep & Ready (it's a POR15 branded solvent), let that dry, brushed off any loose dust and then brushed POR15 literally right over the rust with no fucks given lol