My first attempt at painting a 3d print, the Juggernaut from Dota 2. Turned out quite well, used Acrylic paint for it. Did notice it’s easy to peel off so a coating afterwards is needed.
Did you undercoat the model first? After you print something, you should wash it in warm water to get rid of the slick feeling, first, and then spray or brush it with an undercoat. If you don’t undercoat first, your paint will lift. If you want the paint to be durable, if the model is going to be handled a lot, spray or paint it with either a gloss or matte varnish, to taste.
That experience comes from the wargaming hobby. The first miniature I painted just flaked itself nearly clean in a few days because I took a shortcut and didn’t base coat it.