Gel Coat thoughts
Once an frp boat gets over about 15 years old, the original gel coat can develop surface cracks in any area where it is thicker than nearby areas. This is more common on hard-angle corners but can occur anyplace anywhere. Further, when the resin and hardner are auto-mixed and applied, it is possible for there to be an occasional chemical problem in the material or setting on the pumps.
Production boat builders have this tightly controlled, for the material itself and training of the applicator.
Thickness is usually the culprit for checking in mature gel coat surfaces. You want enough surface layer for lasting gloss that can take repeated UV damage over the decades and be polished and cleaned and still look good. Too thick and you get cracking/checking. Too thin and the next layer (a different color shade) starts to show through before 20 or 25 years.
* in our moorage we have a lot of "classic plastic" boats. Quite a few boats built in the 70's are showing this thinning of the original get coat on their decks where the UV has been deteriorating the surface since the 70's and early 80's. It's common to pretty much every make of boat, including some Ericson's.
As the original gel coat ages and continues to "cure" over time, it slightly shrinks. The flexibility seems to diminish. The area pictured in this thread should not have any structural "flex" to cause a crack, so it seems likely to be a result of aging and application. Note that in the picture the surface nearby has eroded enough to show the substrate color.
Observation: At some point in the getting-nearer future, the deck will need a re-coating. Done on the cheap, most of the fittings are masked off and a one-part epoxy paint is rolled on.....
Done right and not cheap, all hardware is removed, surface sanded smooth, and all fastening holes are over drilled and re-drilled and all hardware dry fitting done.
Then the separate gloss and non-skid areas are shot with LPU paint, several coats. Finally all the hardware is fitted with new sealant.
Then it's lovely for about 20 years or more.
We have a number of boats in our moorage done both the cheaper/faster way and the more-expensive 'Remanufactured' way.
Caution: these comments may be: 1) partly true, or 2) not applicable to the boat in question. That's the problem with observations by laymen. We see a lot over the decades but lack training or actual paid employment in the field.
On the plus side, I have been privileged to repeat-visit a lot of major restorations in high end shops and yards, for over 25 years.
Apropos of Whatever.... that restored/remanufactured Cascade 36 moors next to me.
https://gypsykramer.files.wordpress.com/2015/08/stern.jpg
https://gypsykramer.wordpress.com/2016/06/
Use the Archives icon in the lower left corner look through the work. The owners are still working away on the new interior.
The gloss on that hull represents (IIRC) three color coats minimum and five clear coats over that.
Our boat still looks "great" after a buffing, but is way short of the gloss on the LPU surface on that Cascade. And, while our old non skid collects dirt and mold, the LPU surface next to us is pretty much sealed and impervious to all dirt and stains. i.e. everything just hoses off.
So best of luck on the repair of the boat this thread. Our boat has a few similar small cracks where there were sharp corners in the mold, but we are probably ok for another 8 or 10 years before needing a repaint.
Regards,
Loren
ps: after all this pontificating, there's no harm in adding a layer or two of bi-ax underneath, if you can get to the area. (!)