How to get hardened deodorant out?

[BorgQueen]

Of heavy (cotton pique) work black polo shirts ?
The armpits on all 5 are fully white and hard like cardboard, presumably due to repplication 2-3 times a day while he’s been out in his van all day scrabbling about in peoples’ lofts and cellars in this heat.
I’ve scrubbed a paste of bicarb in with a nail brush and left it 30 mins but the material is still hard after rinsing. They are too good to chuck and they weren’t cheap as they are personalised for the business.

Dye them? Try fairy liquid before a wash, or soak in napisan? To be honest not sure they are redeemable but worth a try.

Vinegar?

Also just googled and it seems there are a number of remedies involving white vinegar and other ideas.

A paste made with aspirin is supposed to be very effective at this.

Try soaking just the armpit areas in a solution of equal parts of white vinegar and water, for a couple of hours.

This helps to break down the deodorant residue.

If that doesn't completely work then make a paste of baking soda and water and apply to the area.
Let it dry completely and then wash normally.

When you wash the shirts, air dry them instead of putting them in the dryer where the high heat will set any remaining residue left in the fabric.

I used the pink stuff oxy stain spray, a couple of times and that improved it.

There's a Dr Beckmann sweat and stain spray. I've used it successfully on light weight fabric
although might not be heavy-duty enough for you.