What gets poo caked on feathers off backside?

I have tried baths with hydrogen peroxide. Her pecks wounds and the feather pulls are healing great. She is eating, laying and drinking well and tolerating the isolation from the other birds great. But her healing backside is caked in poo. Some of it comes off every day with her sitz bath in peroxide, but it is just caked in her formerly fluffy bottom feathers. Does anyone know how to get those feathers clean so she can have fluffy bottom feathers again? What if I let Dawn dishsoap soak those feathers and then did her bath? It gets oil off of birds? I just don't know what to do. She is doing great except for the daily poo that keep caking on her. Any suggestions would be helpful. Thank you! Dawn