A good exterior-grade stain or finish should do the trick.
I'll assume this is wood?...depending on what the current finish is your could go the exterior paint route of use a version of a marine type varnish. Keep in mind "surface" finishes will tend to chip and peel. Where a penetrating deck oil will not. In the case of the later you may need to prep to bare wood.
Both are proper responses. If you want a quick fix, paint with an exterior paint, just know that over time, that will erode, much like the paint on the exterior of the home. It will fade faster if the bench is used often. If you want that look to last longer, then I would do as KMS suggests, prep the bench, use a high quality oil primer and exterior paint for that cover, or use a deck oil. Good luck with your project....