I clean houses and a client of mine told me about CLR.  I always had a hard time getting soap scum and hard water off shower doors and metal.  This stuff made my life so much easier. 

You mix equal portions of hot water and CLR into a spray bottle.  The directions say brush it on or use a sponge, but a spray bottle works better.  Spray it on and immediately start scrubbing with a scrub brush.  You will start to see it come off right away.  When you think you have got it all rinse it off with cold water, repeat if needed and your shower doors will look new.  

It works great on...