Pesky underarm stains

[LBDD]

I want to pass on DD1s school polo shirts to DS if possible as they are unisex and still in good condition.
Unfortunately on close inspection they have obvious yellow stains in the armpits and I just can't shift them. I use biological liquid and have tried rubbing this in before washing, soaking in vanish white powder, hanging out in bright sunlight etc.
Does anybody have a different method? I'm loathe to spend £30 if I don't have to.
Thank you.

I have recently improved some very yellow underarms. I soaked in solution of vanish gold as per instructions. Then I wrung out the shirts and rubbed the neat vanish gold powder into the damp underarms, left for about an hour then washed in usual way. Not sure it can be great for the fabric but I would have thrown the shirts out otherwise and they are hugely better.