How to seal 3d printed parts

Hello,

I've been printing some vases to use in my house, unfortunately as they come off the machine they are not 3d printed. I'm not sure how to seal them, because I don't want to wait for a 15 hour job for a 6 inch model by using 4 walls. A couple of requirements:

-Need to seal inside of vase, not outside
-A lot of vases have small openings in the top, and I can't use spray or brushes to apply the sealant.

Any other ideas? Do you guys maybe know of any liquid sealants that I can pour in and out of the vase to seal it that way?