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.
Right. If the cave were filled with helium and had an airlock, the scientists could still work in space suits.