Mark, I would apply the finish to the cross, before attaching the backer. The way I would finish the cross, I would dip it into a tray of 50/50 mixture of paint thinner and poly. My reason for finishing it this way, is that it will give complete coverage to the inside cuts. I really dislike looking at a cutting such as this, when I can see interior cuts that don't have any finish on them. I find that it makes the cuts always look dusty when not finished properly. Yes, you can always use spray cans or some other type of spray system, but, I find with those types of finish you usually end up with excessive amounts on the top of the item. After I dip an item, I wipe the excess off with a cloth. Then I let it dry for a day, after that I apply the backer board which is already partially completed with the first coat of finish. When I have the two pieces glued together, I apply a final coat of finish.
Len