Mark Panel to Prevent Over-Sanding
Before doing that, though, make a pencil mark over the panel. That line acts as a guide so you can see where you’ve sanded and where you haven’t. This helps prevent you from over-sanding the low spots. 80-grit sandpaper is a great place to start.
Just keep the sander moving and watch as the lines begin to disappear. It takes a little time, but soon enough you’ll sand away all the marks and know that your panel is smooth. When you’re finished sanding, you can cut the panel to length.