Legibility text that is only in the clear Spot UV coating (also called "blind Spot UV") depends largely on the lighting. As nothing is actually "printed" in this scenario, you have to catch the lighting on the coating to read the text. We recommend your type size to be thicker and larger than 12pt Helvetica Regular for blind Spot UV.
If the Spot UV is registered to CMYK printing, then this is much more legible, see: What is the smallest font size I should use?