To prevent a belt sander from tilting near the edge of a workpiece and rounding over the corner, place another board that's the same thickness up against the workpiece. This way, the sander rides across the edge and on to the abutting board instead of rounding it over.
Check your ducts for air leaks. First look for sections that should be joined but have separated and then look for obvious holes. Consult with a professional about repairing duct leaks. Safe duct repairs require a licensed heating, ventilating, and air-conditioning contractor.