You might have catched it just in time .But if you have just prchased the car you can bet its filled with bondo glass .By the time the bubbles appear its rusted through the back ..Any pictures ?
Has to be sanded down, then Phosphoric acid many times untill the rust pits are clean and metal is shiny.
Then Use Epoxy Mastic , then Paint over it...
If its just the arch lip , DIY is possible
The car has accident history as the rear bumper doesn’t sit quite right and the f/n/s bumper under headlight has had a crap repair as it goes hard then softens then goes hard...... is currently soft and the paint has slowly crumbled away......
And, you can clearly see slight colour difference in couple of panels..... I’m colour blind lol but my ex mate used to be a sprayer so I’ve learnt to spot it....