Although I have not worked with IMEX figures, I am assuming that you mean that they are made of the soft plastic that they make soldier figures from. If this is the case, I have had better luck with enamels than I have had with acrylic paints. I would recommend cleaning the figures prior to painting them with either Polly S Plastic Prep or soap and water.
My son gave me some soft plastic figures to paint a few years ago that came with the robot kits he enjoyed building at that time. After a coat of Humbrol enamels, I painted the flesh using oil paints with no problem. I hope that this helps. ![My 2 cents [2c]](/FSM/CS/emoticons/icon_smile_2cents.gif)