How to Clean Yellowed Rolltop Bath?

[Janec4000]

I bought a second bad bath off gumtree. We travelled a really long way and when we got there the bath did look a little yellow but it was dark and I thought it was just dirt as it was sitting in a garage. It's an £800 new bath and taps - Cambridge roll top bath off bath store. Once we got it home and I gave it a clean I saw how bad the yellowing was - and it is bad. It's all over the inside of the bath although worst in the corners of the bath. I don't know what the yellowing is - we live in Scotland so I don't think its a hard water issue. I also don't think its from bleach beacuse it's all over the inside. Any one have any advice or recommend anything to use to whiten the bath? Or is it a re enamel job???