Haven't posted in a minute, but have you experimented with linseed oil, or would that spread to much into the rest of that work? Haven't tried it myself yet, just came to mind. Forgive me I read farther back and see you did. Another alternative might be coconut oil, but in my experiments with making makeup it removes makeup rather quickly so coconut might not be great, or it may. Haven't seen it used with pigments yet.would test it on a scrap canvas with various pigments and mediums first and see what the effect is first perhaps.
Be blessed,
Larry Jackson