You can have the bath re-polished to remove scale & mineral stains, it's not that expensive & is much cheaper than replacing the bath. Alternately, use a specalist paint that is advertised for the job. Check back with the council exactly what the time scale is for the refurb of your specific property. in Cheshire the council re-finished the bath as I first described within a few weeks of us reporting it, it took less than a day, looked like brand new & could be used immediately.