If you do not want to use Balsa Fillercoat prior to Sanding Sealer that is okay.
It tends to fill a little faster if you do but one can use just one or the other.
If one uses BOTH, the Fillercoat goes on FIRST, then the SEALER on top.
It makes ZERO sense to use Fillercoat to fill-in grain that does not exist due to being sealed by Sanding Sealer.
Balsa Fillercoat does more wood penetrating than the Sanding Sealer.
Fillercoat "fills" better than the Sealer, and the Sealer "seals" and hardens better than the Fillercoat.

I sand between every coat of Fillercoat or Sealer except between Coat 1 and 2 of the Fillercoat as coat one mostly just soaks into the wood and does not really "fill" anything.

Been doing it that way since 1977 and it still works well. Finding Balsa Fillercoat now is getting to be a real challenge, but it is easy to make one's own by dissolving plain talcum powder into thinned Clear Butyrate Dope from Sig, Brodak, or new-old-stock Pactra Aero Gloss.

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