How can I clean the grime from this rubberized sealing gasket on my Zojirushi Thermos Flask? This is mostly for hot water sipping, a yogic/Asian practice.

The location of its basically where the lid gasket seals down on the flask lower body. It doesn't have much reason to gather anything.

I'm not sure how water usage can cause such a mark or can build up from something else. Is it just a mark or some build up?

Ps: I don't know exact material of gasket on hand but it's a zojirushi flask if that helps determine a solution - Resolved: It is Silicone Rubber

What is it?
How do I remove it?

I've tried scrubbing it with a tooth brush and dishwasher liquid soap to no effect. This has worked on other areas/plastic parts but did not work here.

I don't have access to an electric dish washer, but if it would help dipping this in hot water with soap or vinegar or something, I'd be open to that as long as it does not destroy the product.

PS: I prefer non-corrosive, organic, or common household solutions.

Zojirushi Thermos Flask Top View

  • Don't scrub too vigorously as the smooth finish will develop tiny scratches that will make cleaning it more difficult in the future. The plastic is soft and flexible to allow the pump in the lid to be effective. What the deposit is exactly, is difficult to identify. From what I've seen, a dishwasher is very effective if you can find someone to include your problem part in their wash.
    – Stan
    Aug 16, 2017 at 2:27
  • Maybe just a scrub is not the answer, and as my question focuses I think I need the right "solvent/ liquid" in warm or hot water. 99% of dish washers in this Asian nation are Human so until I'm back in US of A, I've got find a way to replicate the above. Put in a cloth bag & put in a Washing machine? What temp?
    – Alex S
    Aug 16, 2017 at 2:32

1 Answer 1


Based on your description I would think it might be some mineral from the water that caused this stain. If that is correct and from the looks of it, it might well be iron oxide (or rust in the vernacular).

  • Rust can be shifted with a mix of Sodium Bicarbonate (baking soda) and Vinegar (Dilute Acetic Acid) (some people use lemon juice instead).
  • If not at hand an other alternative to reduce rust is Phosphoric Acid. This is an ingredient of Coca Cola.

Whatever you use I would suggest to soak it over night and have a go at it with the tooth brush the next day. It should come off easily.

If this doesn't work I am wrong and it's probably not rust.

  • Gonna have to try something like this over the weekend.
    – Alex S
    Aug 16, 2017 at 2:33
  • I tried the Citric Acid solution overnight dip & soak - As shown in their How To - youtube.com/watch?v=r8wq4v2BmW8 | I'd like to try your BiCarb + Vinegar solution next. What kind of ratios/ quantity of the solution would be applicable? (Maybe update into answer?)
    – Alex S
    Aug 24, 2017 at 4:19
  • If it was rust the citric acid should have worked. It should have removed lime scale as well. Honestly I don't know what the deposit could be then. You can still try BiCarb+Vinegar of course. I would just sprinkle the BiCarb on first and then drop some vinegar on it in roughly equal proportions.
    – Flint
    Sep 1, 2017 at 10:43
  • I made a paste of BiCarb, put it around the circle, and in a vessel dropped vinegar from top with a lot of fizz, no dice. Mixed it also, kept overnight, no movement yet. I'll try once again with your proportions but not feeling very hopeful
    – Alex S
    Sep 4, 2017 at 17:00
  • Me neither. The citric acid should have worked on either rust or limescale already. I know bicarb and vinegar work on tea stain as well (it looks like it, but you said you only ever used it for water). 'My ratio' is nothing scientific and was more meant like a suggestion.
    – Flint
    Sep 4, 2017 at 18:16

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