Properly apply sevin dust on the chickens, I assure you it'll kill feather lice immediately. If you havnt treated the chickens first, then there's the possibility that the lice infected chickens have reinfested your cleaned out coop. Always dust chickens first, then immediately dust the inside of the coop, nests,roosts. Then repeat dusting everything again in 7-10 days. Also, it's best to always keep the inside of coops as dry as possible.
You can put a good amount of sevin dust in a grocery bag or pillow case. Then put a chicken in the bag with its head sticking out the top of the bag so it can breathe. The rest of the chicken will be enclosed in the bag, then "shake and bake." Remove the chicken from the bag and use your fingers to rub some dust on her neck and head, avoid the eyes and nostrils. Then release her, she'll shake herself off and go about her business. Then grab another chicken and repeat this procedure until they're all dusted.
Then put a small pile of sevin dust at the entrance to the coop. Get your leaf blower and hit the pile of dust full blast. It'll look like a smoke bomb went off inside the coop, and the dust will get into every small crack and crevice when it settles including nests. Make sure that no birds are inside the coop when you do this. Wear old chicken clothes and shoes, and a mask when dusting.
