From here, it looks like a good start would be a decent quailty topcoat, with a bit of build, it appears to be pretty thin whatever it is. Like every one is saying it's probably the water, weather it is forced in or sucked in , it is going to bust any finish. I have seen a wood chip bust a cast concrete slab 6" thick, it's how they split marble- granite etc, wood is just an amazing 'product' good and bad. And after all wood expanding is how they have kept (keep) sailing ships afloat for god knows how long, before that Ark, they reckon. pete