You spelled chemistry just fine. I'm not a conservator but I am a chemist so I can at least spell it. Most plastics are very stable. A very few will outgas at higher temperatures. I can't think of any time that a painted shelf would be preferable unless the fabric is encased inside some sort of barrier. I think the issue with plastic was the fact that plastic bags don't "breathe" and molds can form inside in high humidity climate/areas.
I hope some quilt conservators will chime in and give you their $0.02 worth.