Can you put polyurethane over Sharpie?
Simply so, can you clear coat over Sharpie?
A coat of vinyl sealer will protect the artwork, & you can build up the nitro clear over that. Just dust the vinyl on a couple times before applying a wet coat. Most if not all Sharpies will degrade in intensity & color over time, whether sealed or not. You can count on a couple years before it starts to fade.
Similarly one may ask, can you put Mod Podge over Sharpie?
Since Mod Podge is a water-based product, it may become tacky in humid conditions. An acrylic sealer should be applied over the last dried coat of Mod Podge to make it waterproof and to eliminate the tackiness. And Sharpies are permanent. So if it is dry, you should be fine.
Wipe off all of the excess. Use a tissue to carefully wipe away any extra baby powder or hairspray around the tattoo. When the hairspray is dry, the tattoo should be 'permanent' and won't smear when rubbed with a tissue. This method should make the tattoo last for up to a month.