Getting rid of yellow armpits on white tops?

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Just washed a white cardigan, added vanish oxy action white (or whatever it's called) and used a brightening fabric softener and still have yellow marks under the arms! I have rubbed bio washing powder on the marks before, but no good, and I don't want to wash in bio because I did that previously on another cardigan and wrecked it. It's the same on some white t shirts. Does anyone have any solutions?! Many thanks!

Heat up some vinegar (I use spartprice) in a pan until it's hot not boiling and pour it over the stain.

I leave if for a bit then do it on a hot bio wash.

This is usually linked to anti perspirant containing aluminium. Since switching to aluminium free one I have not had this prob or dh whose shirts were terrible.

You can buy a stain remover called deo go which it utterly fabulous. Dh's work shirts get horrible with sweat and deodorant, the fabric gets stiff and stained but that stuff works very well where all the traditional methods like aspirin or vinegar fail.

Thank you very much for all the tips! I will have another go soon.

DH has this trouble with his shirts so I may check if there is Aluminium in his deo.

Use Bio powder rather than liquid and dry in full sun. Always wash asap to prevent stains becoming ingrained. If necessary wash and dry several times.

Wet the stains and sprinkle liberally with bicarbonate of soda, rub it in so it forms a paste and covers all the stain, add a bit more water if it looks dry. Leave it for an hour or so then wash normally.