Yes, you can over-smooth if you expose the 3D part to acetone for too long. This can potentially cause permanent deformation or dissolve the thin walls of the part. To prevent it, you must monitor the smoothing process carefully. Check the part's progress every few minutes, which is easier with a transparent container. Set a timer and allow the smoothing box to fill with acetone fumes. The duration varies based on your model's size, available space, and the acetone in the tissues.