Hi,
I think I've been using either Mr. Surfacer or Mr. Primer lately (it comes in a square bottle in either white or grey). I can check when I get home tonight.
Other than that, I will often also apply a coat of sanding sealer above where I have placed putty and the where the putty and plastice meet, and then later sand it smooth after the sealer dries. Unfortunately that approach may not be all that suitable, if you are working on a model with alot of surface detail. It also may help to try and use putty relatively closer in color to the plastic that you are working on. As such I will try and use Squadron Green Putty on darker colors, and Squadron White Putty or Perfect Putty on lighter colored plastic.
Pat