How long did you let the urethane "varnish" cure before coating with emulsion? From what I recall, many urethane coatings require curing times of up to a week before they quit outgassing, and the catalysts they contain to promote curing might well fog silver halides. If you can do so, try removing all the gelatin (chlorine laundry bleach does an admirable job of that) and simply let the brain stand in a warm, dry location for a week, then try coating and exposing again...