New HKS high flow exhaust fitted but underside rust

Fitted the new HKS exhaust, sounds and looks amazing, doesn't hang too low either

https://1drv.ms/u/s!Arf-yOsTGQrvjWKg7UvptDPdzp21
https://1drv.ms/u/s!Arf-yOsTGQrvjWVA3kxwVr6bqkW9
https://1drv.ms/u/s!Arf-yOsTGQrvjUeuauW3mEmhAEDo
https://1drv.ms/u/s!Arf-yOsTGQrvjSnfjcY6plBBmEef

However noticed quite a bit of rust from the rear subframe back, I guess I could treat the surface rust myself but the one that concerns me is the rear side member, its squishy under finger pressure

https://1drv.ms/u/s!Arf-yOsTGQrvjS2vNPwtLHW-0xUU
https://1drv.ms/u/s!Arf-yOsTGQrvjS43AXfk6kL38Xk1

I guess I need to go at it with a rotary tool but thinking if its as bad as I suspect, I think its going to be tricky to repair without taking things apart i.e. getting to the back of the side member

MOT coming up so thinking I need to think about addressing this now

Can't see your images bud. Bit sounds like your rear sills might be rotten? Or floor? Will all need to be cut out and welded up. Check out my project thread for details

Sent from my SM-G988B using Tapatalk

ah ok, posted images on my onedrive, may post them on the facebook page instead then, anyway, thanks for that, will take a look at your project thread