One year my students made wonderful giant flower drawings with oil pastels.
On a whim (after a test run on a sample) I coated them with leftover
Polycrylic by Minwax....can't remember if it was semigloss or highgloss. But
it worked great. I buy this stuff from hardware store in a gallon can and
usually coat papier mache sculptures with it. Hope I have the name right.
Polyurathane comes in a brownish can (don't use that) and this is its
waterbase counterpart which comes in a blue-ish can. One child brought their
drawing in a year later for and art show and it was still in great
shape...the paper stays strong and flexible, too.