As DBM said, sanding those wheels "surfacing" them is a MUTHA big time. If you must, and yours are pretty badly pitted then the polyester primer (it is an "undercoat" as "Top coat" is something you leave it in) is the way to go. But don't spray it everywhere, epoxy it and just spray the polyester where you NEED it. Sand it out and then epoxy the whole thing again and spray your paint (top coat) right over the epoxy, as no sanding is needed. (if you follow the manufacturers recommendations for "recoat window")
OH MY GOD does this bring back a horror story in my career. One time many years ago I did a set of Model A wires that were badly pitted. Polyster primed all four, spokes and all, inside and out and surfaced them! HOLY CRAP my figures were raw by the time I finished those damn wheels! I had no finger prints! LOL
You may want to "surface" the metal first with a roloc "surface conditioning disc", I know I would. Get the coarsest one, I forget the name of the color, it's a beige looking one, on an angle grinder and smooth out the metal before priming.
I have never used SPI but I understand it will fill and sand well (Shine may speak up, he uses it a lot) and after surfacing it with the Roloc disc the SPI epoxy primer may be enough. I would do anything personally before I broke out the polyester primer. It is bad ars stuff, a real life saver when you MUST use it, but if you don't, don't, it adds a LOT of work to surfacing.