Repainting solid oak - but with which varnish?

Hello everyone,

I have a shelf in the bathroom made of solid oak, which is coated with clear varnish. Unfortunately, the varnish has come off due to the liquid from contact lenses. Now the spot is lighter than the rest of the shelf. I sanded the spot a little and painted it with synthetic resin clear varnish. Unfortunately, the spot does not darken again. Can someone tell me which varnish will make the wood dark again? Thanks

No paint helps. That's because the wood simply darkens on its own.