Although it resembles a paint scraper, a putty knife has a thinner and more flexible blade, letting it force filling and patching material into holes in wood, plaster, and other surfaces. Steel and plastic putty knives are available in various widths. Home Improvement Encyclopedia
A putty knife is a spatula used for scraping surfaces, or spreading material such as plaster in various construction trades. Widths from 1 1/4" to 5" or 6" are commonly available. Stiff-blade knives, typically 1 mm or .040" thick, are suitable for scraping.