With plastisol we've tried to use a white underbase and it usually looks way worse. It made the silver look like a very light gray and very little "shimmer", same with the gold. Most of the time we PFP a 135s screen and that seems to look the nicest, or at least what our customers like better. Sometimes just one layer looks pretty good to but not as opaque.
We've used a discharge underbase with Waterbased Shimmer on top and that is super soft and looks pretty nice, but just a lot more prep and attention to the variables.
You could probably try a discharge underbase under Plastisol shimmer, just make sure the discharge base is well flashed before putting plastisol on top.
