I did this with my 90 535i last summer. The lt frt fender just had heavy surface rust on the lower inside area by the rocker but the rt. fender was rusted through in that area. You may also find that the front of the rocker/sill has also rusted through as it had on mine. I bought the clean rear section of a crunched front fender from Rigmaster, cut off the section I needed along the lower seam & welded it to my rt. fender after trimming off the rusted area.
After sandblasting the rust off the lt fender I shot the inside of both with epoxy primer (PPG DPLF) before repainting both fenders.
Before reassembling the fenders to the car I drilled a few holes in the rocker/sill ends, pumped Waxoyl inside & plugged the holes. I also coated the inside of the lower rear fender & outside of the rocker ends with the Waxoyl.
If you live in the rust belt it's a good idea to remove the inside wheelwell covers & the fender rear/lower mounting bolts every couple years & clean out the crap that accumulates in there.
90 535im
02 R1100S, 87 R100S
76 R90/S,73 R75/5
Alfa Romeos- 65 Giulia Spider Veloce
& 66 Duetto 1600 Spider