You can’t get dingy white towels white again. I have tried everything on my dingy whites and nothing will make your whites white again.
The dinginess embeds in the fibers and the best hope is to pick a color like tan and dye the towels that color.
It’s like trying to remove sweat stains from t-shirts, everyone says they have a method that works, but I’ve yet to see anyone who can back that up.
So just pick a color get some rit and dye them
If the neighborhood covenants allow it …
Line-dried towels are stiffer, so fabric softener is a good idea then. (Use about 1/4 of what’s suggested and ALWAYS use borax in the wash).
About sweat stains in white t-shirts:
You know, I really never care about them; white t-shirts are worn under dress shirts or to change the oil. Who cares if the armpits are yellow?
That being said, I’ve noticed recently that our white shirts don’t get sweat stains anymore. I’ve been wondering why.
Spread them out on the grass and the chlorophyll is supposed to help, too.
I have an old book called How to Clean Everything It says to make a solution of dishwasher detergent in hot water and soak the offending whites.