You should be fine with painting. The birds might pick at it some, but I doubt that the can eat enough paint to make a difference. Of course if you are in the camp with everything must be ultra natural then this advice will never work. Most paint these days are plastic based. So while I don't want them to eat a lot of it, I have seen to evidence that it harmful to them or us. I wouldn't use really old paint for fear of lead content. It would be very difficult to get or find paint that old that would still be usable. Chickens that are well fed are just not going to spend a lot of time picking at paint either.