[boris]

"Plastic is not static and tends to become opaque over time."

You seem to speak with authority. I am not an expert on polymers, especially the spray-on protective kind. How many are there? Do they all become opaque over time?

How about the clear-coat they spray on cars?

My point is that it should be easy to kill the existing mold, and possibly apply some sort of protectant against future mold attacks. Maybe the spray would have to be refreshed every so often.

--Boris

[WaveThatFlag]

I am no expert either, but I do know two things: It is going to be impossible to remove anything you put on without destroying the paintings, and it is impossible to predict the behavior of any type of plastic over the time period that we are talking about (thousands of years).

[dark_lord]

I think you are on the right track. No, you can't spray anything on. The radiation would work -- but this treatment would probably have to be reapplied every year or so. The question about radiation would be -- would it affect the pigments. Don't know, but maybe.

Another approach would be to simply fill the cave with an atmosphere that would be toxic to the mold. Find a gas that the mold can't survive, pump the cave full of it for a day, and you are done. Of course, you would have to test that the gas would not react with the pigments.