I just thought. The hairline crack I found must have opened / become visible on the back of the body relatively recently ... I grain filled the guitar. I'm pretty sure I didn't see it when I sealed the guitar, and same after first coats nitro.
Ooooh WTF. There's 2 of them slightly offset (look like the cracks you get in wood when it dries out).
Clearly doesn't go right through though body - only seen on the back.
HOW WORRIED SHOULD I BE ?
They are full of nitro (I suspect one has been glued) and one can definitely be felt as a slight ridge - which might just be the nitro building up in/around it.(surface was flat as before).
I guess I just need to finish the nitro and see what happens.