I had better luck holding their butts under really warm running water with less struggle and it was faster and did a better job than trying to use a rag to wipe it away. Then I used the hair dryer to dry their butts.
And I can't believe that I actually did that to a chicken. Now if they would just eat some yogurt so I could see if it would help with the problem - but I think I have some of the dumbest chickens on the planet and they will swim in the yogurt but not eat it.