Clay.... It turns out that synthetic varnishes and polymer mediums were not the boon to the art world as was first thought in the '60s. Art conservators began seeing problems with yellowing, bloom & dead looking surfaces on artworks. It was most apparent in cases of coating.
For platinum printers that are using PVAs similar problems could occur within 10 years. (I have read some of the research done on PVAs by conservators at the National Gallery of Canada.)

I have a bottle of older polymer medium similar to Liquitex that has turned yellow... This would probably not be very noticeable when used as a pigment extender but used undiluted as a glaze it would be. Although manufactures have apparently improved polymer medium formulas there is still doubt and some artists have moved away from synthetic varnishes and returned to traditional resins for coating. I guess each artist must decide if it is an 'issue' or not.... Sorry if I sounded cryptic or alarmist.... Not my intention.