The blackboard I made a few years ago as a quick demo used blued cut tacks to hold the actual blackboard in the frame (simple rebate).
Finally had proof why cut tacks weren’t the ideal solution. Over time, with the blackboard being used by enthusiastic artists, the tacks worked their way loose- a symptom of being a wedge shape. Because of this, they also can’t be hammered back home and have them become tight again.
So they all were removed and replaced with self-tapping button screws. Much more effective, and a simple lesson learned for next time.