Thanks Berta. That's a good idea, but I don't think it will work here. The inside of the cup is on the side that's already stained and finished so water shouldnt affect it
If you can bring it indoors to a lower humidity environment then you can temporarily remove the bowing as it will cause the longer fibers (on the convex side) to shrink a bit and flatten the panel some. Of course, those stresses will return when the board takes on more moisture but it will allow you to at least get things assembled.
When edge gluing panels it is a good idea to take a look at the end grain before glue-up, if the endgrain has a cup shape then the panel will be prone to cupping with any changes in moisture content. In that case it is often helpful to align your boards such that the cupping alternates, that way the panel takes on an often much less severe wave pattern rather than a single severe cup (especially if using multiple narrow boards). But endgrain will nearly always tell you the story of how the board is going to behave when exposed to changes in humidity.
Note that the finished side will still gain and lose moisture, but will do so much more slowly than the untreated side. It can also be helpful at times to seal the hidden side as well so that both sides gain and lose moisture more evenly in the future or if this project will be going into an environment that often sees short term spikes in humidity (such as with bathrooms). Finish never inhibits changes in moisture content, it just significantly slows the rate at which moisture is gained and lost. Even otherwise stable quartersawn lumber can temporarily bow and cup if only one side is sealed and the other left unsealed since the unsealed side is subject to more rapid changes in moisture content due to humidity changes.
When thickness planing lumber to thickness it is good practice to take roughly equal amounts off each side of the board to help reduce cupping as you expose interior wood, that may be at a different moisture content to the outer wood, to your shop's ambient humidity. Do not remove all the necessary thickness at once but rather leave some thickness and then wait a few days to a week before final thicknessing to give the wood some time to equalize to your shop. This way the lumber is more likely to remain flatter during construction provided your shop is not exposed to extremely severe swings in humidity from one day to the next. This will help to reduce the likelihood of cupping between the milling and assembly operations. It also helps to allow your boards to acclimate to your shop for a couple months by moving them into your shop ahead of time if you tend to store your wood supply somewhere other than your shop.
It is also very helpful if your shop is humidity controlled, such as using a dehumidifier, as this maintains a much more stable humidity level year round which greatly reduces wood movement not only from one day to the next but even one month to the next. As someone who is much slower than most due to disability I would never be able to practically kill and assemble projects in the same day, but with humidity control I can literally mill lumber in one season and assemble in another without any significant wood movement or changes in dimensions that can cause headaches during assembly. Humidity control also means very little time spent dealing with tool and cast iron surface rusting plus the shop is much more comfortable since the air is drier and sweat can evaporate during the warmer months (which also makes air conditioning much more comfortable).
[edit: convex side up of course]
