How to clean tea stains from a mug with steel interior?

I had great success cleaning tea and other food residue with chlorine-free bleach (sodium percarbonate). Put a couple of tablespoons in, soak with hot water and leave overnight. Then scrub a bit, and everything comes off easily.

Yet another way of cleaning coffee/tea stains could be to use a denture cleaner, one of those tablets that fizz if thrown into some water. I personally really like this, as it's simple, safe, and can be used not only for mugs for also for e.g. thermos bottles or reusable plastic/glass bottles with a much narrower opening.

Just boil some water, fill the mug with it while it's still boiling hot, set it in a sink just in case it overflows with fizzing, and drop in one or two tablets, then leave overnight. If the mug/bottle doesn't overflow/is done overflowing, you can use an oven mitt to carefully take it out of the sink and place it somewhere safe if you need to use the sink.

The next day you can usually just dump out the water and rinse once or twice, but I tend to do this more regularly, so I've never quite had a buildup like in your picture. In your case you might have to repeat, in which case you might not just want to rinse but also use a cloth or brush to see if you can remove more.

If your mug is unpainted on the outside, you can clean it in the dishwasher. However, if it is painted, then you can't. Here's alternate ways of cleaning:

What worked for me:

Put 1 tsp of dishwashing detergent in the mug, then fill it with 90C or hotter water. Let soak for 1 hour. Scrub briefly with a bottle brush. Pour out. Repeat if necessary.

(note: this works in a Contigo mug, which is stainless steel. It is not a good idea for aluminum or some plastic mugs)

What many internet sources suggest:

Make a baking-soda and vinegar mixture in the mug. Scrub, and rinse.

I've had good success with, strangely enough, fizzy Vitamin C tablets. I stumbled across this by accident when I notices a badly stained teacup was sparkling after I dissolved one of these in my cup. This has become my go-to way of cleaning stained cups.

Tea stains in ordinary glazed china mugs are easily removed with a citric acid + hot water + scrubbing with a non-stick sponge.

Let the citric acid solution "soak" in the mug for 5 - 10 minutes before scrubbing gently.

Citric acid can be bought at most pharmacists, veterinary products stores and dairy products stores.

I clean my stainless steel travel mugs using Barkeepers Friend and a non-abrasive scrubbing sponge. Barkeepers Friend's main ingredient is oxalic acid. It's recommended by e.g. AllClad for cleaning their stainless steel pans, and it works like a charm. For hardened stains I usually sprinkle the powder and then cover it with a wet paper towel and let it soak like that for a while, but it's usually not necessary.

If you're in N. America you should be able to find it at most grocery stores and also big box home improvement stores like Home Depot. I have no idea if it's available internationally, but I would assume that it or something similar should be.

The classic way to deal with tea stains is bicarbonate of soda/baking powder! Mix it with a little water to make a paste, apply liberally, leave to sit for a while and rub off.

I usually scrub with salt - just salt and a little bit of water. For the actual scrubbing I would use one of those sponges with a 'sole' of rather hard plastic felt - put the salt in, fold up the sponge with the scrubbing side out, so you can insert it into the mug and use something like the handle of a wooden spoon to push it around. It works surprisingly well.

I've cleaned some mysterious stains using CLR as a last resort. https://clrbrands.com/Products/CLR-Household/CLR-Calcium-Lime-and-Rust-Remover

However this stuff has it's own collection of nasty chemicals and you will need to rinse the bowl a dozen times with warm water and agitation. Also let the item sit overnight to dry, and then rinse a couple more times.

Curiously, there are multiple different packagings/labellings around the world for the same product.

Just for completeness, we have used liquid bleach to remove tea/stains from ceramic and plastic mugs. But never tried on metal ones. It might ruin your metal. :-)

Note: We have used diluted, water-treatment-level, sodium hypochlorite bleach (standard from supermarkets). Let it soak for a while. Then clean thoroughly. Here "water-treatment-level" means that it is a pure solution of sodium hypochlorite in water, without additives (e.g. perfume). The bottles we get have clear labels, either "can be used to disinfect water for drinking" or "cannot be used etc.".

I don't think anyone has mentioned enzymatic cleaners, AKA "oxy-" cleaners. One major brand name is Oxy-clean. The tea stains are protein, and enzymatic cleaners do a great job of breaking down proteins. I've used enzymatic cleaners to great effect in removing coffee and tea stains from glassware and mugs. I've made the inside of Thermoses and coffee carafes look like new with zero scrubbing.

One major caveat: If there is any printing on the object you're cleaning, like measurement marks on the side of a Pyrex measuring cup, the enzymatic cleaner can take it right off.

For stainless steel, Barkeeper's Friend is great, I send that recommendation if there's a reason to not use an enzymatic cleaner.

In general I have found tooth paste excellent for cleaning or polishing stainless steel - the cheaper the better :) The cheap and nasty brands that make your eyes water are often the best. Also good for chrome and any surface that you might scratch.

I’ve had great success using bottle cleaning tablets. They usually contain baking soda, hydrogen peroxide, and citric acid.

https://www.today.com/shop/people-say-these-tablets-make-their-bottles-look-brand-new-t173018

Just did it the other day on a coffee thermos to remove a very stubborn black spot and now it looks very shiny and brand new.