tips for spraying primer

O.K, that's what I wanted to know. If you've hit bare metal in spots, then it's time to stop sanding and put on another coat or two of high build primer.
What I would do in your situation is check the P-sheet for that U-Pol primer and see what you can use to thin it down a little. Usually you can reduce your primer by about 10%, and in this case that's exactly what I'd do. If you spray it at the recommended mixture, you may get bridging of the primer - ie, it will be thick enough it may just bridge the solvent pop craters instead of filling them. If you thin it about 10% with reducer or Xylene, as per manufacturer's recommendations, it should be thin enough to flow into the craters, not just bridge them. I'll see if I can find the U-Pol P-Sheet to see what they recommend for thinning and at what percentage.

And as Mike said, make sure you don't have any silicone products or things like WD-40 in use in your shop while doing any type of body work. It plays havoc with any and all paint products.