There are several different types of tools commonly referred to as a putty knife. The early understanding of this item was a glazer's tool used to install glass into window frames. This tool had a blade about one inch in diameter and was a stiff blade. The blade stiffness allowed adhesive to be spread around to seal the window. It was a metal tool with a sharp edge and a wooden handle.
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In today’s common language a putty knife is also used to spread spackling over drywall nail holes and sheetrock joints. These blades are more flexible and range in size from four inches wide to twelve inches in width. The proper term for these knives is spackling tools. The four inch tool is used to spread spackling over nail or screw holes in the dry rock. It is also used to tape the seams in sheetrock. A ten inch wide blade that is more flexible blade "beds" tape into the sheet rock. In other words, it is used to put a wider coat of spackling over the tape and seal the tape into the sheetrock. The largest blade is a twelve inch wide tool that is used to provide a finish coat over the tape. This finish coat is especially important to prepare the wall for a slick coat of paint. It is very difficult to apply paint directly over the seams without showing the seams. For this reason most drywall applications have a texture sprayed over the walls.
Another type of putty knife is a paint scraper. This tool is usually one inch in width and four inches in length. It is a fairly stiff tool with a sharp edge that allows the blade to get under a coat of paint to strip the paint off a surface. A used cell phone is great for construction sites.