UV-Transparent Plastic? I am looking for a type of plastic, preferably in the form of a sheet or film, that is weatherproof, durable, flexible, and both transparent and UV-transparent. Most films I've seen so far are treated to be UV-resistant, where can I find untreated plastics for purchase?
Planning to use this plastic in order to cover a solar panel, without inhibiting energy output.
Did you consider PMMA?

A 3 mm Plexiglas G UVT layer seems to have a transmittance > 80% at $\lambda = 300~\mathrm{nm}$ and above.