What I use -
Using an All Purpose Cleaner, a stiff detailing toothbrush or a regular toothbrush.
The below pictures aren't the worst I've seen, but this method works the same on heavier stains too...It just might take more dwell time and/or a few times to remove all of it.
Here it is before cleaning
After Cleaning with QD
After cleaning with APC and toothbrush
Let it dry completely before applying dressing to make sure that all of it was removed.
After applying a coat of Dressing
This method has always worked great for me.