Our second baby is due in the next few weeks — we haven't found out the gender and didn't for our first either. For the first baby I made a blackboard from an old window I found so that friends and family could vote on what they thought the gender would be. I didn't have the correct tools and spent many, many hours hand sanding and removing old paint from the window.
For gender guessing blackboard #2, I really needed to get it done quicker and most of the time I'm chasing toddler #1 around. I went to Home Depot of purchased a ONE+ Cordless Random Orbit Sander. It was a breeze to get the window to a usable state. In total it took maybe an hour and 45 minutes.