Badly stained as in from hard water/rust/calcium buildup?
Easy Off oven cleaner in blue can--yep seriously I cleaned apts/rentals and that is what I had to resort too sometimes & for mineral build-up use straight CLR
Like Lynn I use that too. Amazing how well it works. It does smell so spray it on, turn on the exhaust fan and close the door behind you. Go back in about half and hour and scrub and rinse well.