The Duplicolor has always worked well for me but there have been instances when as area remains soft for longer then I'd like.
I too have noticed that if you put on too much material at once, the primer will take a very long to to completely dry.
I spray on a medium wet coat, let it dry for about four hours and hit it again with another medium wet coat.
I will then let this gas off for about a week before sanding. Just because the material is dry to the touch, it does not mean that it has dried all the way through.
Nobody has mentioned it here, but I wet sand the primer. This will keep the paper from loading up and makes for better cutting action.
Very messy but speeds up the sanding process. Also, add a few drops of dish washing soap into a bucket of warm water. Again, this will help to keep the sand paper from loading up.
Larry, Buttafucco Stunt Team