Well this tray art was much more time consuming than I thought! I went with Michael's advice and got an all purpose epoxy since one side was masonite and one side metal. It really seems to be sticking very well. I wouldn't try to hang on it though like that guy in the hard hat.
I painted out the rest of the tray with black paint and let it overlap the Pastelbord a bit, then I sprayed the whole thing with a clear coat. I need to do some touch ups, another coat of clear coat and it is ready to go!
Unfortunately I had the wrong day written for my Open Studio Still-life class. It is on Mondays at 10 am and will be starting this coming up Monday. Sorry for the inconvenience.