What coat can I apply on a gallery canvas (before painting on top of it) to prevent it from yellowing over time? I'm planning to leave some parts of the canvas unpainted.
If you want the canvas to be raw it will discolor but if you want it to look raw and are ok with covering it, then use a good quality acrylic gel medium. Golden makes several, and if you use a matte gel and cover both sides of the canvas you will not have any yellowing as it will be insulated from oxidation. Matte, is suggested because a thin coat is almost impossible to see. Gloss will show itself.