Scott,
1- The fact that the crazing is in a circular pattern is normally indicative of a void between the gelcoat and the substrate and the gelcoat caving in on it causing the crazed ring. This is not uncommon in hand laid hulls. Anyone who has worked in a mold will know how easy it is to roll some glass and have it lift as the roller passes over it just enough to cause a void. On our brand new 2005 Catalina I had about seven of these voids that I found and were fixed before commissioning. Because many boat owners don't realize how important it is to survey a new boat these voids are often not caught until a crazed ring shows up or the boat is for sale and the surveyor finds them. This is where the resin infusion methods for building boats really shine! Voids on resin infused boats are nearly non-existent.
2- Sound out the area with a small brass or phenolic hammer and listen for changes in tone. You will likely sound a different tone inside the crazed circle.
3- If the soundings indicate a void it needs to be injected. If the void seems deep from the sounding you can slightly thicken the epoxy with some cabosil. If it is not very deep un-thickened resin and a syringe (West System sells them) will work fine.
4- It is unlikely that the gelcoat crazing continues into the substrate so it is mostly cosmetic. Crazing in the gelcoat will not normally let in much if any moisture content to the underlying laminate. I have had my meter on many crazed ares, Cape Dory is FAMOUS for crazed decks, and the readings over crazing are no higher than non-crazed dry areas.
5- Fixing the crazing can often make it look worse so consider a pro or just living with it.
Fixing Yourself
1-You would need to cut a v-groove following the craze lines with a Dremel. This opens it up and gives the gelcoat tooth and prevents it from re-appearing. It is important that the edges are beveled and have a gradual taper to them.
2- You would want accurate color matched gelcoat in both waxed and un-waxed or some PVA in a spray bottle.
3- Route the groove and fill with slightly cabosil thickend non-waxed gelacoat to near full but not full.
4- Lay on a thick layer of waxed gelcoat to a thickness higher than the diamond pattern.
5- Use a set of jewlers files, chisels and sanding blocks to re-create the diamond pattern. This will work significantly better than the method where you make a mold but is tedious.
6- This is done AFTER you have done the injection to repair any void that may be there so don't forget to put a dab of gelcoat over the small injection hole. Usually two holes and a 1/8" bit is sufficient for a small void.
Unless you suspect the underlying lamination is compromised I would not bother with thinned epoxy, caulkings or other band aids as there is little chance of further damage. If it is a void I would address it so water can't become trapped between and "pop" the gel in a freeze..
