Gelcoat damage and structural damage are different repairs
Gelcoat is the pigmented outer layer. It is thin, it is not what holds the boat together, and it is what you see. A scrape from a dock piling that has taken the shine off and shows white underneath is usually gelcoat only. Press on it. If the surface is solid and the damage stops at the colour, the repair is a fill, a fair and a polish. Nothing structural has moved, and the boat is not taking water through it.
Structural damage goes into the laminate below, and sometimes through to the core. The signs are different: the panel flexes under hand pressure, the crack has a raised lip or a step across it, or you can see fibre strands rather than smooth resin. That repair means grinding back to sound material, laying new fibreglass in stepped layers and rebuilding the surface afterwards. We establish which one you have before committing to a method, because the two jobs are not comparable in scope.
Stress cracks around fittings
Fine cracks radiating out from a cleat, a rail base, a ski eye or a transom bracket are stress cracks. They are telling you that the fitting is loaded harder than the laminate under it can take, or that the backing plate is undersized, missing, or has crushed the material behind it. Filling the crack and matching the colour hides it for a season. It comes back in the same pattern, because nothing about the load has changed.
The fix is to unbolt the fitting, look at what is behind it, and repair the substrate before touching the cosmetics. Often that means a larger backing plate, or a solid fill where a through-bolt has compressed a cored section. Star-shaped cracking in the middle of a flat panel is a different thing again and usually means an impact from the inside, such as heavy gear shifting in a locker under way. The pattern of the cracking is the diagnostic, not the crack itself.
Punctures, and why the core is the urgent part
A puncture from a rock or a hard grounding looks alarming and is often the more straightforward repair. What matters is whether the hole has reached the core, the balsa or foam sandwiched between the inner and outer fibreglass skins. The skins are waterproof. The core is not. Once water has a path in, it travels through the core well beyond the visible damage, and the boat grows heavier and softer over time. Tape over a hole gets you home. It does nothing about that.
Around here the two usual causes are dock contact and the granite shoreline. Lake Winnipesaukee has at least 264 islands and a great deal of rock sitting just under the surface, and granite does not give. A glancing hit at speed can open the skin over a surprisingly wide area. We map the extent of the damage before cutting anything, because the wet area is almost always larger than the hole, and new fibreglass laid over saturated core fails from the inside out.
Delamination, and how it is found
Delamination is the skin separating from the core, or one laminate layer letting go of the next. There may be nothing at all to see. It is found by sounding: tapping across the panel with a small hammer or a plastic mallet and listening to what comes back. Sound laminate rings sharp and consistent. A delaminated or water-saturated area goes dull and flat, and the change is unmistakable once you have heard both. We sound the whole panel, mark the boundary in tape, and work to that line.
A wet core in New Hampshire does not sit still over the winter. Water in the core freezes, expands and drives the skins further apart, then thaws and refills the larger void. Repeat that through a season of freeze-thaw cycling and a soft spot the size of a dinner plate becomes a soft deck. That is why we would far rather find a wet core in October than in May. The repair only ever gets bigger across the cold months.
Colour matching, and why the hull has to be dry and warm
Gelcoat is matched by eye and by mixing, not read off a code. A hull that has spent summers on an open lake has faded, and it has faded unevenly: the topsides more than the area shaded by a cover, the horizontal surfaces most of all. Matching to the original factory colour produces a patch that is visibly newer than everything around it. We match to the hull as it is now, mix test samples, cure them and compare in daylight before committing to the repair.
This is shop work. Polyester and epoxy resins need a dry surface and a stable temperature to cure properly, and gelcoat wants the same. A repair laid up on a damp hull in a cold open yard traps moisture in the laminate and cures soft, which means doing it twice. We work mobile at docks, moorings and driveways around the lake for plenty of jobs. Laminate and gelcoat are not among them; those boats come to Mirror Lake.