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I've used a Stanley 10-499 quick-change retractable utility knife forever, for all the usual tasks - sharpening carpenter pencils, opening boxes, scoring drywall, and cutting building paper, nylon line, composition shingles, and insulation. It slips easily into the holder in my tool belt, and spare blades stack in a plastic tray that swings out of the handle at the press of a tab. Blades are installed and removed by shifting the slider all the way forward and then pressing a button on the nose of the knife while slipping the blade in or out. I've stuck with this...

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