Oil paintings are pretty tough. If they're stored in the dark, they may darken somewhat, a change that's reversable when they are returned to normal room lighting.
A painting stored directly in contact with any material (or another painting) poses something of a problem. It's not inconceivable the common cardboards could be reactive in time with bad effects because of their acid content. Practically all plastics are chemically active to some extent, off-gassing volatile elements (e.g., anything you can smell) and containing plasticizing agents that may react or migrate in the presence of other materials that also contain solvents and volatile elements . . . such as paints and varnishes, so plastic sheeting could cause problems when directly in contact with a painting. Much depends on the age of the painting, the type of plastic, and the length of time in contact.
Between bonfires, I store my "backlog" paintings vertically in wooden racks, no contact one with another, or other materials. Museums store paintings in shallow dust-proof cabinets faced with glass under ambient light,, hung separately as on a wall, whether framed or unframed.
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