I was at my PPG dealer yesterday and asked about thinner for the Nason Select Prime 2K I use and they told me there wasn't any, but that's just for that kind and I didn't ask about others. Like Jeffl says, primer is supposed to be sanded and I've never heard of not sanding it prior to painting. All of my primed surfaces are sanded with 400 grit wet... and when done, it's smooth as glass.
On a related issue, I always thought that sealer was only recommended if you were painting over anything other than a completely primed area like a repair where part of the panel is primer and the rest a color. Isn't the purpose of sealer to prevent bleed through of anything that could contaminate the finish?
