Addressing Tough Plumbing Stains
Different materials can stain plumbing fixtures from time to time (i.e. drains, sinks and toilets). Some may be easy to clean while others can be extremely difficult.
To begin addressing tough plumbing stains, the very first thing to note is what not to do: never use metal scrapers, spoons, rods and other tools that can damage the toilet. Metals are harmful enough to damage the coating of any toilet.
Oxygen bleach is a powder that can be poured into the toilet and allowed to rest for thirty minutes. This product is effective in cleaning light to average stains.
For hard-to-clean stains such as lime, hot white vinegar solution mostly does the job. But because vinegar is only a light acetic acid, it is not enough to remove harder stains. For this, muriatic acid mixed with five parts of water can be used. Pour just enough solution up to where the toilet’s water level is.
Source: How to clean tough toilet stains