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you are aware that the effects of darkening are more environmental reactions (as in UV) than the finish itself...
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Arm-R-Seal will not darken as it has UV inhibitors, but it will start flaking when the inhibitors wear out. A light sanding and a light coat will fix that¹. Don''t bother with a clear poly (or anything else over Arm-R-Seal) it is less durable than Arm-R-Seal, it is just a waste of time and money. GF HP has the best UV protection of any finish that GF offers and is more scratch resistant than the others although not as chemical resistant as Arm-R-Seal. I would expect Arm-R-Seal to last >>7 years unless in direct sunlight. GF HP is a good topcoat over shellac. It cures quickly and sprays easily. Arm-R-Seal doesn't spray well if at all and is slow to cure.
¹ All finishes will degrade over time as the UV inhibitors wear out. GF HP has a dual inhibitor system, one protects the wood and the other protect the finish.
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If you like the look of Arm-R-Seal there is no need to put anything over it and lots of good reasons not to. It is very durable. It will not darken over time. Joe has it right. Whatever change happens will be more due to the underlying wood reacting to UV light than aging of Arm-R-Seal.
I take it you want as much grain pop as you can get, and a light finish where both the wood and finish do not yellow or darken over time. The best recommendation I can offer for that is a coat of SealCoat shellac followed by GF's HP Poly. Personally, however, I would use Arm-R-Seal. The picture above is my reason why. I made those railings 6 or 7 years ago. I saw them about 2 years ago and they looked like new. Arm-R-Seal does not break down over time, as was suggested, unless maybe the piece is going to sit in direct sunlight every day. I don't have that problem where I live. It's as durable as any other interior varnish, and better than many.
John