depends on how much past 90 you are talking about. Normaly I'd use a belt sander the length of the edge and do it freehand. I've posted these pics before but here they are again. the before and after using a belt sander and 80 grit followed by a random orbit sander with 100, 120, 220 grits to smooth things out. Make sure to use a respirator if you have MDF and don't do it in the house.