Did the joint compound work to eliminate the perf line on the foam? I have that on some of my tombstones also. I did a test run on a piece of scrap with one coat of drylok and even though the line is tiny you can still see it in some lights. On the attached picture it's at the top of the piece. Not sure whether a second coat would help or not. I tried to fill the cracks with wood glue on one tombstone but I think it just made it worse.