The best affordable
exterior grade clear
coat may very well be--paint base.
I happened across a web page years ago, where a guy experimented with untinted
exterior house paint base as a clear coat for outdoor projects.
I didn't find it just now in a quick web search, so I'll go by memory as best I am able.
for the darkest
colors is clearest. The number
for the darkest will be the highest. So, one brand would be paint-base 5. Another would be paint-base 4, whichever is the highest number for that brand.
The clearest untinted paint-base is not perfectly clear--it will have a slight hue to it--but should be unnoticeable if only a few coats are applied.
I recall that the guy said the pastel bases are not clear at all.
I remember that he used several different polys/varnishes on scrap pieces and one or two got the untinted paint-base and it was surprising how quickly the polys began to develop cracks when left out in his back yard.
As I recall, his reasoning was that outdoor paint-base is made for weathering and has UV protection built in and can flex a little with the wood--it is made for that, after all. And, it lasts longer than most other wood-coating solutions.
I wish I could give credit its proper due, but I don't know who he is.
If anyone finds that site, please post it here. I will add it if I find it.
Meanwhile, who's up for a little experimenting? Use some scrap, first.
Edit: I found this statement and many like it--"Epifanes Clear Gloss Varnish holds up well but also requires yearly attention to stay very nice outdoors."
A lot of people swear by Sikken’s, as well.
Saw a post recommending Bristol
. I haven't used it myself. It's claimed to last 4-6 years.
Ready Seal is said to be a good performer on decks.