Yes, regardless of material selection or if the sign is sub-surfaced painted, the photopolymer layer must be protected from UV rays with a top coat (painted color or clear coat)
Nova recommends using an automotive-grade acrylic polyurethane, ie. Matthews acrylic polyurethane or equivalent.
According to Matthews Paint Company, it is specified that clear coats and/or paints should have the same amount of coating (4 mil wet/2mil dry).
A Wet Film Gauge is the most accurate way to measure the thickness of the paint coating when it is first applied to a surface or substrate.
Yes. Regardless of the application (Interior/Exterior) a top coat must always be applied over the photopolymer. This is true for all sub surfaced painted signs, signs with sub-surface digital graphics and materials that incorporate color or patterns in the base substrate.