Tumeric is good, but can only use in warm/hot weather, personally I have never used anything to toughen birds skin via washing, it should gradually get tougher through proper conditioning and massage IMO.
Bignasty like this time of year I wash them using warm water and keep them indoors until they completely dry off. In spring and summer cold water straight from the water hose is fine. Ideally after you wash them you want to put them in the sun but in winter you can only do so much. Steaming a bird you can do it daily .
I am living In a cold country and most of the time the weather is bad
But my youngsters are free range birds so they are always playing in the rain and getting wet
But they never get sick at all,,,
When wash them I use termeric and ginger mix , mite killer liqued water .