Most finishes will be degraded by the sun. Spar varnish is a finish used for many exterior applications, but will need periodic repair. Meaning sanding and recoating. Might last two years, might last less.

Polyurethane for floors is tough enough, but allows sun damage to the wood. And is tougher to re coat since the poly chips easily. Meaning more sanding before re coating.

Or, use clear paint. Which protects the wood color, but will probably wear faster. Clear paint? Yup. With Sherwin Williams brand, it is the ultra deep tint base with no added color. Goes on milky white and dries clear.

@HandyMac, thanks so much! One follow up question for you... Of course I will be sanding these pieces before resealing, but assuming that there will always be some residual sealer left over, is there any risk of the clear paint or spar varnish lifting or otherwise not tolerating the previous sealer?

Thanks again!

Sorry for the lag, had a short hospital stay interruption.

After sanding, wipe well with a rag dampened with a little alcohol. That should remove any contaminants.