My latest project has a steel bath, fitted around 15 years ago. It looks just fine except for mottled black/grey staining in the middle, around where your feet would be if standing under the shower. The staining was there when I bought the property but I figured it was due to poor housekeeping. However after battling with various lotions and potions it does fade somewhat but doesn't go away. Shape wise it's almost butterlfy shaped with a thin (around 1 inch wide) line going down the middle which is perfectly white.
I don't want to have to replace it so am looking for advice on any cleaners that might shift it or indeed what might be causing it and how I can prevent it getting any worse!
I do use a bath mat but that's not making any difference. Thanks much!
Hmm, based purely on its location, it sounds like limescale to me. If you've tried all the cleaners to get it off, it may be that is all you can do. It is entirely possible that repeared cleaning / scouring has damaged the surface, thus leaving a permanent mark.
Ary.
-------------------------------------- ***Do not, I repeat Do NOT feed the Trolls! *** *** All Hail the mighty hamsters! ***
All Hail the mighty hamsters! 
