It specifically states "Do not thin." on the website for the product.
So I would say you are to brush it on, like they say.
[link:minwax.com/products/protective/helmsman-direct.cfm | Minwax Helmsman Spar Urethane]
(See the bottom line here)
Maybe what you want is a different product, specifically for spraying?
[link:home.att.net/~paul.edmonds/|Sonny Edmonds ]
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Helmsman Spar Urethane can be thinned 50/50 with mineral spirits for spraying. However it is important that you:
1. Keep it constantly stirred or the 2 will separate in the spray can/gun.
2. Make darn sure your wood has 15% or less water content, or the mineral spirits will cause the excess moisture to come to the surface in the form of whitish splotches, droplets, or other whitish looking junk that doesn't sand off readily....you'll ruin your wood.
3. Mix only what you need at the spraying moment... and not for the whole job.
4. The mineral spirits will take away, a tiny bit, from the luster or gloss of the urethane alone, so the finish will not be quite the same as if you brushed on the pure urethane.
5. Any mix your have left over, don't attempt to save it. It has no shelf life.
6. I suppose you know to clean your tools, well, after spraying.
What are you spraying? Do you need a finish comparable to a floor finish...if so, then consider Varethane. The water base Varethane dries really fast, so 1) by the time you finish spraying the first coat, you can immediately begin again with the second coat, then the third, etc. and 2) clean your tools immediately after use.
Helmsman spar urethane is not the product for finishing outdoor furniture, however I finished a swing, protected under a patio, and it has held up for 10+ years. Consider tung oil or a marine finsih for a more durable finish in a sun exposed scenario.
How slight is the arc? If it is a couple inches over 80 or so for a queen headboard, just cut a thin strip and glue/clamp it on. Up to 1/4 of an inch thick will flex onto a shallow curve very easily....