The old Krylon Triple thick seems to be the fav.

Problem with sealing a BG is that there are many things out there that will do the job, but not many that have been done 10 years ago to prove they stand the test of time.
Polyurathane works but yellows over time.
Clear coat works but you got to test the CC against the inks used for compatability - tons of CC products out there - not all compatible

Krylon - been on the market for ages. tried and tested