I'm not sure if there is anything special about diesel soot. My first thought is Snobol Toilet bowl cleaner. Just wipe it on and wait about a minute and wipe it off with a bucket of water. Wear gloves!!! After clean throw some speed glaze on there or whatever you use as a "wax" or "non wax wax". If you don't the pours are open and will stain easily with the next splash in dirty water.

Mike Hill
N20 1005