The gelcoat on mine had tiny cracks all over. Somebody had painted it without taking care of the issue. All the cracks printed through the paint. I can imagine someone giving a boat like this a radical treatment by grinding off all that gelcoat. When I refinished it, I decided to spend the time to enlarge the most visible cracks and fill them. Then I used an epoxy coat and fairing compound on top of that. So far, no indication that any cracks have survived.
I have no idea what the cause of the cracks was. Some were localized and clearly related to some damage. Looked like a bullet hole from a '22

(rather inexpertly patched). But mostly they were all over without pattern so I think they must simply have been shrinkage.