Rusting at bottom rear of rear door frame

Evening all,

I know there is probably a whole world of pain here but what are the treatment options. Underside of car is/seems very solid.

Thanks Richard

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.

Water trap and moss lover. Best remove the plastic capping and good rub down. Might get away with filling and respray. It's the plastic capping trims cause the heartache.