"Unprimed" refers to something that has not been prepared or initialized. It usually implies that the object or situation is in its original or basic state before any additional actions or processes take place. Full definition
The artist has alternated long periods of working in either watercolor or oil, specializing in controlled pours and stains on unprimed canvases early in her career. (arttable.org)
She had begun painting on unprimed canvas, which allowed the pigment to seep into the fabric. (washingtonpost.com)
During the 1950s he produced still lifes indebted to William Scott (also no relation) and then at the start of the following decade, when he represented Ireland at the Venice Biennale, more landscapes, often of bogs, which at last found him beginning to discover his own distinct timbre through the use of unprimed canvas into which tempera paint sank. (apollo-magazine.com)