The solid content contains a resin component (A) and a filler (B). The resin component (A) includes at least one of a fluorine-containing copolymer (a1) or a silicon-containing copolymer (a2). The fluorine-containing copolymer (a1) includes a fluorine-containing segment and an acrylic segment containing no fluorine or silicon. The silicon-containing copolymer (a2) includes a silicon-containing segment and an acrylic segment containing no fluorine or silicon. The filler (B) has a mean particle size falling within a range from 10 nm to 200 nm.