Guys, I had something similar happen to my boat.
Just forward of the cockpit, it wet out clearly and I added fill coats as usual. When I drilled holes for the bungee hold downs, I noticed the glass weave starting to reflect light where ever I applied pressure to the deck. This has happened in this area where I scraped. I can push with my hand on the area and no change and then scrape it and it will create the "diamonds".
The strange thing is that it did not happen behind the cockpit. It was the same batch of epoxy, same glass, etc. etc. I may have to strip off that section and redo it, but what did I do wrong?

Don T

: It certainly is not outgassing as that creates bubbles.
: What happened is that your fibreglass did not wet out for one or
: other reason.
: The fibreglass was fine for the rest, so it isn't the wrong type of
: coating on the glass.
: As Bill says, there can be a myriad of reasons for this.
: JohnK