I'm hoping to make a Medic cosplay between now and April... as long as my current TF2 project doesn't kill me. xD Did you make a pattern yourself or did you find one?
I'm sure you can paint it with another type of paint. If the old paint is keeping other paint from applying.... you could try using nail polish remover on a paper towel to remove it. I'm not sure what you used to make the pack itself and how it reacts to chemicals but it's worth a shot!