How to get rid of tea stain from thermal mug? ~ Part 2

I've tried using lemon juice to get rid of the tea stain on thermal mug but it doesn't work. This time i decided to try to clean with baking soda.

Baking Soda (Sodium Bicarbonate)
80g for RM 0.80.
I bought it at the shop that sells bakery stuff.

thermal mug that full with yellowish tea stain.

I tried to google to see whether I can find any instructions on how much baking soda to be used to clean the thermal mug. unfortunately, I can't find any. Since there's no instruction, i shall just try it out myself. I used two teaspoons of baking soda, mix with warm water and soak it over night. The next day i used a sponge to clean it. Some part can be cleaned easily but some still not able to clean. I think maybe i didn't use enough baking soda. Therefore, i used another 3 teaspoons of baking soda + warm water and soaked for another night. This time, i am able to clean every part easily.

see the difference? the before and after?
baking soda really works but make sure you used
enough baking soda.
(5 teaspoons of baking soda + warm water
for a 0.5 litre thermal mug)
try it out yourself!!