I'm am far from a professional so take whatever I say with mounds of salt. Those lines are caused by the natural shrinkage of the boards and the way they were milled. If you want a smooth flat surface you'd be best off removing the board (which will save your sand paper too, as it looks like you're destroying those stainless steel deck screws $$$) and sending it through a planer.
Is your deck pine?
mkgcb12 t1_iskpphu wrote
Reply to Deck sanding help! Orbital Floor Sander leaving lines by AsianAssHitlerHair
I'm am far from a professional so take whatever I say with mounds of salt. Those lines are caused by the natural shrinkage of the boards and the way they were milled. If you want a smooth flat surface you'd be best off removing the board (which will save your sand paper too, as it looks like you're destroying those stainless steel deck screws $$$) and sending it through a planer. Is your deck pine?