Frog Juice is a product I used this summer to clear coat a mural. It is
graffittii resistant. 1 spray can would probably coat 2 trash cans twice. a
bit pricey though your principal is right but make sure those
cans are not painted with an oil base paint as is usually the case a primer
might be needed
http://www.enduragloss.com/Merchant/Tools/fluids_sealers.htm