Water Poly vs Polycrylic vs Water Spar

I’m trying to decide on the best finish for flag I’ve made which is impregnated with water based stain. I can’t quite decide which would be the best choice. I’m trying to avoid any yellowing, so I know urethanes are inherently inferior in that department, but would seem to be the most durable. The flag my be partially outdoors, which made me lean toward the water based spar urethane, but I desperately want to avoid as much of the usually beloved warm glow as possible. Polycrylic tends to appear slightly “plasticy” in my opinion, but I’m not sure about water poly.
Which one would yellow the least, while offering the most protection? Am I missing any options?

Most waterborne finishes, including those that contain polyurethane resin, do not have an amber effect and have a "colder" appearance. The amber/yellow doesn't generally come from the resins in the finish, although some will amber over time anyway. This is why most oil based products have an amber hue where waterbornes "in general" do not, at least in the short term. For your application, you really do need to use an exterior rated product for the UV protection, etc., but anything made of wood that lives outside is going to change color over time. And that will most likely be toward the grey end of things rather than yellow.

One option is Target Coatings EM9300 polyester waterborne which is exterior rated and very tough, especially if you use the crosslinker. Finishes like that are best sprayed, however, although you can hand-apply them if necessary.

EM-9300 is exterior rated but it's amber so that's not going to work for the OP. About the only exterior rated non-yellowing WB product I can think of is 2K Poly, aka automotive clearcoat. Not cheap, but really durable.

John