I've just read another link and they said 1. glass polish that you can purchase at automotive stores; 2. oven cleaner, not put on direct, but put on a sponge wipe on and then wash off (recommend wearing gloves) or 3. shaving cream leave on for 30 mins then wash off.
If it's the shower curtain itself, it's probably a combination of soap and minerals. Wash separately in hot water with bleach, dry only until slightly warm and wrinkle free, and re-hang.
If it's a shower door, I use baby oil or vegetable oil with a scruffy sponge to get rid of the mineral deposits/soap scum. Rinse with hot water then use Comet or Ajax. Rinse again. Then clean with a window cleaner.
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