The inner fenders should be loosened and the mud behind them washed away, you will be surprised how much is deposited there by the cowel drains and wheel spray. I changed the WS washer reservoir and I was stunned at the amount of dirt behind the fender liner. I cleaned it and covered the area with that anti rust spray, CLEAR FILM.
I also used it on the inside bottoms of the doors, water on the exterior of the rear side windows is diverted to the inside of the doors and drains out of the door bottom, weird.
spray the underside of the car with the hose to fem over built up mud and silt, it is best done when the car is still damp from a drive in the rain.
Also clean the mud that accumulates on the top of the fender lip, this is a primes spot for rust holes on cars in wet climates like the UK. Also wash the areas were the inner liner screws are as mud collects there.
It is best to do this after the car has been driven in the rain as hose water just rolls of the dried mud. Look at the rear wheel area as well.
Keep that mud off the car and rust problems will be minimized.