Painting Soft Plastic

I don't paint figures much, but I do have some skill in that area.  Most of my figures are plastic.  Some plastics are so dense that they don't hold paint well, so it is important that you wash off the mold release.  Normal warm water and dishwashing soap is fine for this duty.  Paint likes a rough surface to adhere to and a dense smooth plastic won't hold paint well.  There are three things to try.

Once you paint your figures spray them with a coat of Dullcote.  Dullcote is the easiest method to try and the least effective. But sometimes it works.  White glue, Elmers glue, watered down a bit, with about 75% glue, 25% water can be brush painted on the troops and will dry clear. It can be used to hold the paint on.  It works pretty well.

Minwax, the stuff to stain wood, can be used too, you simply dip the figures into the can and allow it to dry. It comes in many colors. Find an almost clear color, or use it as a technique to weather the figures. I have even posted that weathering technique on Bunker Talk in the past as a way to weather unpainted figures. Twilight Creations Zombies! were dipped in two different shades, a few years ago with good results. It acts like a shading or a wash to fill in the folds with a bit of color, it also seals the model to hold on the paint.

Both the glue and Minwax can obscure very fine detail so a careful inspection and brushing out with an expendable brush are recommended while they are still wet.

http://www.minwax.com/

Try these techniques on a bit of sprue if you still have any, but make sure it is the same brand and same figures, so the plastic is the same. If you don't have any sprue left, try it on a couple figures and see how you like each technique.

You can also prepare your figures in advance by using a plastic primer spray paint.  Rustoleum and other companies make them, they are sold in hardware stores, like OSH and others.  They work pretty well too, and when combined with these finishing techniques make the paint stay on reasonably well.