I personally have never painted my cage before, but this article here seems like it could help you:
However, after reading through various articles, make sure that when painting, you wait as along as possible to allow all the fumes to leave.
If it were me, however, I would personally spend the extra money to buy another cage due to the risk of fumes from paint, the hard work it takes, and the fact that there are no risks in buying another cage. This is just my opinion however, so you may do as you like and I'm not trying to say that what you're doing is bad or anything like that. I just wanna warn you and make sure that you know that there are risks.