For deck furniture refinishing protects, I like a spar varnish with a sealer for exterior wood because it is a smooth hard finish, but requires more maintenance than penetrating wood sealers.
A customer had a wooden picnic table she bought many years ago. Most of the varnish had worn off, but the wood was still in good shape. I told her any finish applied to the table would help protect it.
A varnish finish is desirable because the surface is smooth just like indoor furniture. It is easy to clean and will not let a splinter into your seats. There are many types of outdoor finishes; some provide a smooth varnished surface, while penetrating finishes leave the wood grain exposed.
Note: I blatantly use the word varnish to describe all finishes that leave a smooth hard finish. Spar varnishes, urethanes, polyurethanes, polyesters, all provide a smooth finish. I prefer spar varnish for exterior wood because it is more flexible than urethanes. When dealing with exterior wood, flexibility can be important.
Part of your decision between a varnish finish and a penetrating sealer is the matter of future maintenance. Just about any finish will look good all summer, even if you use the least expensive product in the store. A quality product will look good the following year, but what about the third year? Will a light sanding or cleaning be all the prep-work required for another coat of finish?
A varnished surface needs to be sealed from all sides to prevent moisture from penetrating the wood. The finish is doomed once moisture penetrates into the wood. As the sun draws the moisture out of the wood, the varnish checks (hairline cracks open up) and starts to peel.
If the table is sanded and refinished every other year (many manufacturers recommend every year) the beauty can be maintained with little other work. Remember to thin down the first coat with 10% mineral spirits when varnishing bare wood. This makes a good primer that helps reduce peeling. You can also give the bottom of the table a heavy coat of the thinned varnish to better seal the underside.
What if you want a finish requiring a little less attention? Then look at penetrating wood sealers. Most still seal the surface allowing for easy cleanup after the BBQ. Usually they have a life expectancy exceeding two years and best of all they do not build up to a solid finish that can peel when the finish or color is worn. All you have to do is clean, sand if needed, and reseal. No stripping or sanding off of the old finish is required.