Let's have an honest conversation about skid marks in underwear, because someone has to. If you're still wearing a pair that's seen better days, there's only 1 thing to do and it's not to keep washing it and hoping for the best. The solution is to buy new ones. Full stop. I'm not judging. We've all held onto gear longer than we should have. But there's a point where the right move is obvious and this is it.
What I think keeps blokes holding onto old, worn-out underwear for too long is a combination of habit and tightarsery. Skid marks in underwear aren't a permanent condition of the garment. They're a signal that the garment has done its time and needs replacing. Quality underwear that's regularly washed and properly cared for doesn't accumulate permanent staining. When it does, the underwear is telling you something. Listen to it.
A mate of mine is the most frugal bloke I know. He'll nurse a pair of undies well past its useful life in the name of not spending money. I finally convinced him that the cost of quality Barramundies underwear spread across 2 or 3 years of wear is less per day than a cheap pair replaced every couple of months due to exactly the skid marks in underwear problem he was dealing with. He ran the numbers himself and switched. He hasn't gone back.
In my opinion, if skid marks in underwear are a regular feature of your laundry, the answer isn't a better stain remover. It's better underwear. Barramundies makes quality gear that holds up to regular use and regular washing without becoming a permanent record of your digestive system. Buy new ones. Take care of them. Replace them when they're done. That's the whole system. It's not complicated.