Painting knife

A very thin, flexible tempered-steel blade, usually elongated, it is similar in use to a Palette Knife, but more delicate and therefore more suited to precise placing of color on a ground. Painting knives did not appear until the latter half of the 19th century, although various other paint-spreading devices such as spatulas and palette knives, date back to primitive painting. Source: Ralph Mayer, "A Dictionary of Art Terms and Techniques"