Thanks---I have had good luck in the past by adding a mix of water, XIM extender and floetrol and even a little water polyrueathane if it is not the top coat.As stated, that's the key is to get the right needle and nozzle for the material. I'd suggest using some Flotrol added into the mix. The reason why is if your spraying lighter coats (less material) the HVLP is going to blow air as well, drying things out quickly. I've had issues with PC before with touch-ups where the sheen doesn't come out the same, even though it's from the same bucket. So..it's crucial to get all the passes overlapped and done correctly the first time. Extend the drying time and it will all level out nicely. PC is awesome stuff! I'm using 10 gals of it now for some drywall in a house we're building.
That Rustoleum water poly is a good product to spray through and HVLP but the satin finish is too opaque, the semi is fine.