PVA/sawdust filler BEFORE sanding?

[CardboardPlastic]

Hi, my local timber merchant has offered me sawdust if I want to use it for floorboard gap filler with PVA. Can I fill the floorboard gaps with the PVA and sawdust mix and THEN sand the floorboards afterwards? Or is there a reason I shouldn't do this? I'd love to do the filling before the sanding for logistical reasons too boring to go into here!