The same materials painters use would seem logical, since cyanotype is a painters' pigment. A thin coat of clear Krylon is a favorite of many people. I doubt if it would have much affect on permanence, however. Air can still reach the image through the back of the paper, and light can reach it from the front, although UV may be somewhat attenuated. Coatings will protect against mild mechanical damage. Cyanotypes seem to be a bit funny about permanence. Some fade quickly, but I have seen at least one (at the San Francisco Museum of the Legion of Honor) that was over 100 years old and looked absolutely new. (AFIK, it was not coated in any way.)