Had some issues with spi matte clear on new wood doors at work. Hazing, spotting. Biggest problem probably was moisture & sap levels still too high in some of the new stuff, which the matte flattners didn't play nice with. Never tightly wrap or cover a clear coated item, can cause hazing. Older wood much more stable. Never had bubble issues & if I would have, they would have sanded out & recoated.
Recently did fogged glass repair on doors I spi coated several years back. They lived a horrible neglected life of heavy morning dampness, dirty, mildewed, & horribly beat by afternoon Fla sun. First glance to untrained eye looked like needed refinishing, a harsh cleaning with denatured alcohol had em looking less than a year old. What looked like partial finish failure near the joint areas was wood movement, finish itself fully intact on close inspection.
For your barrels, if you stain, give plenty of dry time before clear. A scuff sanding after 1st or 2nd coat & 1 or 2 more coats of euro should give your project an heirloom quality finish.
Go for it.
pouring resins not so great for other than flat surfaces. You're probably best to check with the manufacturer about anything other than pouring. most likely awfully thick to spray & not sure if solvent thinning would be acceptable.