[drcoastline]

They look to have already been surfaced to me. The red arrows look to be chatter (snipe in Zekes post) from the planner. If it were me I would put some 80 grit in the Porter Cable and sand uniformly across the slab just until these marks are gone. After that you are refining the sanding marks. After 80 move to 100 or 120 then 180 or 220. After each grit thoroughly vacuum the surface to remove all sanding dust and any loose particles from the sandpaper. You can wipe the surface with mineral spirits every so often to see your progress. This will highlight any sand scratches that are in the surface and dissipates quickly. You will want to vacuum before wiping the mineral spirits on.