Testor's Model Master Acryl Acrylics
They are a type of acrylic enamel (Not be be confused with oil based enamels that take paint thinner to thin and clean, as these are indeed water based, but are NOT cheap paints..as they bond to almost all plastics, metals, even some rubbers and most all vinyls and for sure, polystone!)
I use them almost exclusively, as they do not need primers before, or coats of clears after, to bond to surfaces, or to stay on. For your information, I am repainting my Jaws maquette, which is a polystone sculpt, with the Testors Acryls.
I never use cheaper craft paints, as they often are larger grain pigment (the color particles in the paint) which do sometimes leave a minute grain pebbling on skin and other fine surfaces, and use cheaper carrier or base (the liguid the pigment is immersed in) that will not adhere well to some surfaces, and the Testors acrylic enamels WILL bond well to most surfaces. In fact, I have had to remove heads or parts I have painted, from finished works, (my own...I assure you) and have really had to manhandle the part to get it loose, heat it, whatever, with no loss of paint from the surface.
There indeed may be a better paint for polystone, but I use the Testors. Swear by it. I should sell the stuff!
(I am curious as to what paints customizerwannabe is using, and on what, and if it needs primer and coatings to bond...as I apparently haven't had his luck with them myself...)