I’m using a combination of acrylic ink and liquid watercolor on YUPO. By the end of each project I have areas that have pooling or are just a bit thicker with color. I’ve let them sit for about 4 days but the thicker areas are still wet - not to the point of moving when you shift the paper, but enough so if you touch it there will be a fingerprint. How long will it take to fully dry? Is a fixative required?

As YUPO is a synthetic, non-porous material, any paint that's applied to the surface will need to dry by air and will sit on the surface of the paper. The acrylics will also take a bit of time to dry due to its acrylic polymer composition. Much like painting on a canvas the thicker build-up of paints will take some time to fully dry. You might inquire with the paint manufacturer about the average dry time for their acrylics. However, environmental qualities will certainly factor into this, such as humidity.

Once the painting is completed you can use aa sealant to protect your work. Here's an article on our forum about the recommended sealants:
http://www.legionpaperforum.com/ask-an-expert/post/2713566