Robertc44 is right that grinding works better for major concrete problems, but light sanding can fix small pits after the concrete dries all the way. On my last project, I smoothed driveway edges with 60-grit paper and it handled the roughness fine. A grinder is best for deep flaws, though. What sanding method did you try on your planter box, grant_price72?

Learned that lesson the hard way too... thought I was so smart skipping meals, then ended up buying a ten-dollar granola bar at some tourist trap. My past self was so dumb sometimes.