I have 8 hens and 6 of them appear to have the same thing - 2 of which are particularly bad with more than just the base of the tail bald, these are a Black Rock and a Bluebelle and they're the 2 hardest to catch too

One of the 8 has also been laying a soft shelled egg for 2 months or so despite my ensuring ad lib access to oyster shell grit - it seems to explode on landing and I have been concerned at the prospect of egg attacks tho fortunately the proper shelled ones appear safe from pecking. I am getting 6-7 good eggs a day mind so they're not at deaths door, just embarrassing to look at and not know what to do next

I have tried diatom on birds, diatom in the dustbath and in the coop including nest boxes where I know they'll sit in it, red mite powder/spray, anti-peck spray, adding more fresh food, increasing/decreasing the corn ration in case it was too heating, you name it

all to no effect.
This thread is great timing - I have some Spot On pour on for cattle/sheep/lambs left that I've used on the ponies for lice for years but I'd never thought of it for the chooks. Issue one was getting 5 drops on each bird so I hope none are overdosed
Assuming success, is there a withdrawal period where eggs should not be sold/used by me? And when should any improvement be visible - do new quills start appearing quite fast?
Hope I've not poisoned any of them by tomorrow, it's been a bad time recently and I could do without feeling guilty but I am at my wits end what to try next so I've just leapt in to do something

I'm so glad it's not just my hens tho..