I agree the true marine spar varnish is very hard to beat, but like Jim said, it's expensive. It also takes quite a few coats to get a good build for max protection. But I've used the untinted paint that Rennie mentioned, and got very good results; long life, nice appearance, and quite a bit less expensive.
Here's the article that turned me onto this trick if you think it might be of interest. Be aware, it's a little dated...and oil based finishes are getting harder to find. But I just did pick up a can of SW All Surface oil based exterior (they call it "ultra deep base" now) for my next effort, it looks a little milky in the can but dries to look much like an oil based varnish. I had previously used the Olympic product, but can't find the oil based version any more. Interestingly enough, some of the outdoor acrylic (waterborne) exterior paints (like SW A100) in the deep color base seem to work as well, though I don't have long term experience with them just yet. Anyway, one other point...whatever you choose skip anything "poly". Like Jim, I'm surprised what you did lasted this long, they just aren't good outdoors. BTW, that SW product I referred to is stock
#6403-25775 (gloss), the first store I asked had no idea what it was (the guy asked his Iphone about it
, Siri admitted she didn't know anything about it either). The second store I took the stock number with me, it was normally about $23/qt.