Hi,
I found a great product for 1/72 soft plastic : I'm using vallejo premium clear varnish as a primer. First I carefull wash the figures, then I apply the vallejo premium clear varnish (in my case, using my airbrush), and then I can use anything I want.
Usually, I do this
1 carefull wash the figures, to remove any grease or release agent
2 airbrush (you can use a brush) spray of vallejo premium clear varnish
3 (optional : coat of non diluted PVA glue on weak parts of the figures (weapons, legs, arms) to make the figure more sturdy)
4 then, you can paint the figure like any hard plastic, resin or metal figure : primer, paint, varnish,etc...