Clear Polyureathane works great. Look on the shelf at the hardware store. It comes either as a liquid or in a spray can. For extremely hard to reach areas you can shoot the spray into a straw or if using the liquid type of can, pour some into the area and let it flow around. There are several brands. Mini Wax is one.

BTW polyurethane is the same base that is used for some brands of matching colored sprays for monocote or ultracote.

Ernie