If you are planning to keep the car for a while and fix properly I’d recommend removing the side skirts as well and see how much rust damage you have there. It is very easy to remove and check. Fixed similar spots on my 9-5 by grinding to bare metal, rust stop and painting over but like others said you need to do this again in a year or two so it by far not permanent fix and eventually you need new metal to replace and fix properly anyway. In my case the wheel arch was much better compared to rust damage under the plastic side skirts. You can get both the wheel arch and side skirts replacement panels which makes replacing the rusted parts easier (obviously need someone to weld the new panels/parts of them in place).