Depends on what mites you are treating? With bare patches it sounds like lice. What you are likely to encounter are Northern Fowl mite, which are very tiny, eat under the skin leaving red scabby areas and we treat with Frontline Spray (unlicensed for poultry) from the vet at a maximum dose of 1.5mL per Kg bodyweight. It stays in the skin for up to 3 months. Red mite and our preference is a steam cleaner on the coop, creosote on the perch ends to trap any getting off the chickens and potash in the soil bath to kill them, but there are loads of chemical or drug based alternatives. For lice we use Barrier Louse powder and in extreme cases one squirt of Johnsons dog or cat flea spray (unlicensed for poultry) as any lice passing through the spray area will be killed, coupled again with potash in the soil bath (not too much or the chickens will get respiratory problems).