I use a five inch, random orbit air sanderThe pad is brand newI use mostly, but not exclusively, 3M aluminum oxide open coat, 150 - 400 grit paperThe swirls (J-tracks if you will) are more noticeable depending on the species of wood in useFor example, on soft woods, the swirls don't show up until the final catalyzed lacquer is appliedOn hard woods, the tracks show up right away, but are a huge pain to removeComments.Opinions.
Probably a combination of both plus the pressure you apply while sandingYou didn't mention what size orbit the sander has but a 5mm orbit is notorious for leaving tracks as opposed to a 3mm orbit which seems to do betterScrapers and planers are great for pre-sanding when the surface is unevenHowever, they leave scrape marks especially noticeable on figured wood, like walnut burl, for instanceStick to the higher grits for prefinish sandingHope this helps.
