Hi Guys,
Thanks for the tips. I don't think my Stynylrez froze, but I hadn't realised it needed to be mixed so thoroughly. So time to break out the Badger stirrer as suggested. Thankfully, it was test piece and not a models I primed.
For Valenstich, Vallejo polyurethane primers are good, but you've got to let them cure thoroughly as they will peel if you sand them before they are fully cured. In fact, it's probably better to treat them as non-sandable primers and use them as a finishing or final primer coat once you've filled all the gaps and holes in your models and sanded everything smooth. They self-level beautifully, and you can mix them any Modelcolor or ModelAir paint to provide the shade of primer you want.
Best wishes,
Will.