Not to be bearer of bad news, but if it shows there the inner wheel arch behind the liners will be worse.
Best bet strip out arch liners, clean and identify all rot. Cut out rust, make repair panels and either weld in or get a mobile welder to pop over to weld it up.
Worth taking plugs out and pumping sills with wayoyl or similar while you are at it.
Re-coat inner arches and underside with suitable underseal or flexible paint.
Replace wheel arch liners.