I had the same thing. Took my bucket (DP97) to a model shop and the guy said there was no need for primer. I'm about 60% through painting (I'm airbrushing with water based acrylics). I didn't use primer and it looks just fine so far. You must sand it, though, to give the paint a key to hold on to.
My next helmet I probably will prime, just to give myself a clean blank canvass to work on.
I guess it's all down to personal choice.
I'm not a paint expert, however, so I defer to the greater knowledge of anyone else who has good painting experience.
Si