The thing is, it is the wrong time of year to be spraying PP on the girls (it is VERY cold outside to get them wet). I have some, and have used it on THEM when they had mites, and it didn't kill the mites. In fact, it was a few days after I treated them that I found mites on ME (I never even saw them on the chickens). I just squirted one squirt under each wing, and one squirt at the vent, though. A dip in the stuff might do it.
I was just treating them to keep away the mites. Also at the time, I was spreading HEAVY amounts of DE everywhere in the coops. I churned the shavings every day with the DE in them. The dust blowing around was incredible. And it didn't kill the mites.
Mites can kill chickens- suck them dry. So it is important to get rid of them. It is a personal decision whether to use chemicals.
I used Eprinex pour on the first time, which also worms them, but simultaneously dusted with Sevin dust. So I cannot vouch for Eprinex alone as a treatment (but others have said it works).
I have also used the permethrin dust they sell in the feed store. But in some areas of the country the mites are resistant to permethrin. Both powders are very toxic and you must take precautions- mask, gloves, long sleeves, take shower afterwards, etc.
http://ohioline.osu.edu/vme-fact/0018.html
This website will give you some info. if you wish to peruse it.
Overall, I have to say that the Sevin dust works absolute WONDERS. You must retreat them as the above website recommends to get the hatching eggs.
Also- I have my chickens on a treatment schedule with Sevin dust to prevent further problems, as they had infestations last Sept. and then again in Dec.
They are free from problems now.