Nope, polyurethane and varnish will need to be completely stripped off and re-applied every 10-years and that's after re-coating every few years. Clear Stains don't seal the wood to stop decay, only Solid Stains do and they of course get rid of the wood's natural beauty. I learned on a single 2-car garage door, the hard way and finally found THE solution.
IF the fence has regular Sun exposure. I now only use One-Time Wood Protector (
) due to it beating polyurethane and literally everything else in everyway. One-Time's very liquid so it goes a long way and goes on very easily and very quickly. It never needs nor can be stripped, just VERY quickly re-apply every 7-years and the container leftovers have an unlimited shelf-life.