I did this job on our ML 270 in May last year (nice weather & outside).
Removed all the crud from under the wheel arch lip, scraped & hosed down.
Grind/rub down the rust to bare metal.
Fill holes if req, sand & flat back, mask up.
2-3 coats zinc primer, flat back, 2-3 coats of grey primer, flat back.
2-3 coats top colour.
2 coats lacquer.
I found some rubber trim on fleabay, think it was for Mini guttering, stuck it with Evostick Sticks Like S**t to the inner lip.
Masked up again, sprayed Hammerite/Waxoyl Underbody Seal on the upper inner wheel arch lip.
Fortunately, our car is Black, so the rubber trim & Underbody seal can't really be seen.
5K miles later, car parked outside - no garage - all okay, fingers crossed.