Something interesting happened the other day. I pulled out the box of passenger cars I had set aside a few years ago for my Rocky Mountaineer. Wouldn't you know that the two Ultradomes I got from Shapeways at the time are White Strong Flexible. I completely forgot.
That comment about them being like sandpaper is absolutely spot on and the material is difficult to sand. The sides of the prototypes are smooth so I skimmed Tamiya filler on and sanded down to 1000 grit. I sprayed them with four coats of Tamiya white spray bomb and only now do I have a smooth uniform white surface. They just kept soaking up the paint.
Now that I have a surface I feel I can work with I'll wet sand them and work on the finish coats but boy one thing is for sure, I'm never ordering anything that will be visible again. Except maybe a stucco house.
I think an automotive primer, as mentioned, might have been a good option but I was afraid my Polyscale paints wouldn't stick.