It was after a few tips on Reddit that I thought I'd try sulphamic acid on crusty sweat stains. It totally totally works. I've tried lemon, vinegar, baking soda, Dr Beck Ann's, etc and nothing has worked as well as this stuff.
I used this www.ebay.co.uk/itm/99-5-500g-PURE-Sulphamic-Acid-Sulfamic-Crystal-Descaler-Rust-Remover-/133729879848?mkcid=16&mkevt=1&_trksid=p2349624.m46890.l6249&mkrid=710-127635-2958-0 but there are loads of suppliers as it is often used to descale coffee machines.
I tried it on a tshirt of my husband's that was about to go in the bin, so it didn't matter if I ruined it. I put about half a cup (ish?) dissolved in a sink of warm water (using rubber gloves). I stirred the tshirt in and left it for several hours. I scrubbed off the loosened residue and then let it soak for several more hours. Then I scrubbed it again, rinsed it thoroughly in the sink and then put it through the washing machine. It came out absolutely fine with nearly all the gross build up gone.
Since then I've used it repeatedly on my own clothes as well as his. I've done it on tshirts with plasticky designs as well as the more integral print and it hasn't affected the print on anything. It hasn't faded colours. It hasn't affected stitching. It just gets rid of gross deodorant/sweat build up beautifully. It does take a while because you have to soak for so long.
Highly recommend. Worth a shot (especially on clothes destined for the bin if you are nervous).