I think I've figured out why I can't seal my barrels. I just need a stack of cured hides to seal the thing, and I only have like, three to my name, lol!
And I now know that I need 50 salt to turn 5L of water in a barrel into salt water and 50 lime to turn 5L of water in a barrel into limewater. I didn't realize that before, and I never noticed it on the Wiki or in the in-game guide to working with leather and preserving foods.
So you folks can just disregard this post!