Bareback chickens bother people more than they bother chickens. The feathers will grow back after they molt. You can do the above advice if you want, but you can do nothing and it will be fine too. Nothing will make the feathers grow back before the molt in the fall.
I am surprised that you have not had this issue before since you have had chickens for so long, but maybe you never have had a rooster, let along multiple roosters before.
Some chickens feathers are not very strong, they are more prone to barebacks than other hens, regardless of the rooster. Some roosters are harder on chicken's feather than others. Another poster, bred for stronger feathers, by only hatching eggs of the good feather birds, and eventually got rid of this problem.
Mrs K