How do I permanently get rid of this discolouration in my bathtub? (I have hard water)

Vinegar

Anything with hard water, just soak with vinegar.

You can even fill a ziplock bag and tie it on a faucet.

If your water is really hard, pour vinegar into that little tube in your toilet too, the jets get clogged easily and that flushes them out and prevents them from blocking up. If they are blocked, the vinegar will eventually open them up if you don't regularly.

Obviously water softener if you don't have one, but depending what your waters like you may have to do more.

An alternative to vinegar is citric acid. You can buy it as a tub of crystals so it's much cheaper because you aren't paying for water. It's great because you can add more teaspoons to water to make it stronger than vinegar. Plus it doesn't smell bad like vinegar.
- can this also be used for rust removal on metal?
  Ive been using vinegar so far
- Where do you find citric acid? I've been using white vinegar in a jar in my dishwasher which gets rid of all water spots and cleans better with my hard water. However, I'd like to find a place to purchase citric acid crystals without relying on Amazon.
I think if you sprinkle a little baking soda in there that could help with some of the more stubborn stains but vinegar alone works really well. I recently cleaned all the hard water stains off my sliding glass shower door with it.

I don't know how to fix your problem, but please post this image to one of the UFO communities.

Oddly my thought as well. Lol

CLR usually works quite well for stuff like this. It's a spray cleaner. I think it stands for Calcium, Lime, Rust remover.

Just a word of warning on this - I’ve had CLR ruin countertops and sinks by accidentally leaving it in contact with them for a long time, make sure you’re diligent about wiping up rings from containers and any drips - learn from my expensive mistakes!
Also test it in a hidden spot first to make sure it doesn’t dull the finish or something.
Edit: IMHO give the vinegar suggestion a go first, much less caustic
I use CLR too (lots of tannin in my well water). Requires some elbow grease but works pretty good

Important question: What is your tub made out of?

Some cleaners may destroy it if you use them.

So it looks like ya got a couple of issues here. As the peeps mentioned, acid - weather in vinegar (preferably stonger like 30%+ if available) or chemicals to take off the lime deposits is a first start. Second it looks like the top layer(s) of the enamel are damaged and or cracked which is why you are struggling with it "coming back and not completely going away". There are fixes for this, but replacement is usually recommended if this is the case.

Thanks for the info, do you know what the fixes would be, short of replacing the tub?
- I'm far from a Profi in the field but I have looked into it before. There are companies that will do it either in home or take it to them, but often it is expensive. There are many home repair kits available, but the paint on / fingernail polish kinds work poorly. The sand the whole tub down and put a new layer on are better... But the cost and work typically don't justify it. If they are just top surface damages, you could try a higher polish sand job on the tub and see.

I can't see anything, but I second vinegar.

Apparently the trick to cleaning is to use acids and bases where appropriate. Acid will work here afaik.

I'm curious as well, I have the same thing.

I helped a friend with a bathtub that had similar stains , but it was almost black. I have a high grid polishing sponge for email, but it was too much, so i whipped out car polish and it worked.

Honestly, have you tried magic erasers and some elbow grease? That’s usually my go to if regular subbing fails.

Best of luck!

Assuming that is an enameled tub. Try Bar Keeper's Friend (powdered not the liquid) dampen tub lightly and add a decent amount following up with a light scrubbing, letting the paste to sit for several minutes. It may need to be repeated or increased scrubbing. The slurry will probably require dampening it a bit to work it around. I find it removes iron and hard water stains. I have used toilet bowel cleaner but it is too harsh when cleaning the tub nekked! Like others have said, a water softener is the way to go. Our filtration system is currently offline waiting to be re-bed.

Since I tried the liquid soft cleanser, will the powder be enough of an improvement to warrant purchasing?
- For the 3 or 4 dollars I feel it is worth having on hand regardless. I use it quite often when cleaning pots and pans where Bon Ami doesn't do the job as well.

I would think you would need a filtration system to soften your water or it will just keep coming back.

I’ve had luck with citric acid on removing scale, but be sure to test a small part of your tub before using any new product to be sure it won’t damage the finish.

Unfortunately we do have a water softener but for some reason, there's still buildup.
I've been seeing improvements with powdered bar keepers friend but it takes several passes and still comes back unfortunately.

Bar Keeper's friend will get that out, I use it when things get especially grimy. I've used it on a similar tub before that was pretty gnarly with discoloration worse than that.

It is very abrasive though, be careful not to scrub so hard you damage it.

Mr. Clean magic eraser

Dunno if this will work, but white vinegar kills a lot of stains.

Bleach does the job for me. Add water and some bleach and let it stand for bit before wiping and rinsing.

And if that doesn't do it, try the trick I learned for cleaning the cutting boards at Subway. Lay down paper towel over stain, pour straight bleach on the paper. Let it stand overnight. The paper keeps the bleach in place and it won't dry as fast. A little harsh, but it works.
- Haha, what? First, why does a cutting board need to be spotless? Second, no way you used bleach on this and then cut food to serve to customers... I hope. Anyway, good cleaning tip for non food handling items.