Hull Archaeology

signal charlie

Well-Known Member
Staff member
Found this fun post and photo today from a few years back, educational:

Fun photo I found in the Yahoo files, kind of a good look at the anatomy of fiberglass and gelcoat. The hulls are made by spraying gelcoat into the female mold that was first coated with a release agent, it is king of like hard, thick paint. That is the crusty blue stuff in this photo. Then fiberglass cloth is laid on the inside of the gelcoat and it was wet out with polyester resin. The resin is the brown stuff coating the cloth. On this boat a lot of the resin has ablated off, leaving fuzzy cloth. The fiberglass and cloth should have a clear appearance, if they are cloudy, then that are has been crushed and the resin is fractured away from the cloth. So a lot of time the gelcoat can be repaired without messing with the fiberglass. If the fiberglass is crushed or degraded to the fuzzy cloth stage, then the fiberglass needs to be repairs as well.

Fiberglass anatomy.jpeg

So the fiberglass is what floats the boat, and the gelcoat is there to protect the fiberglass and make the boat look pretty. Gelcoat is basically thick paint with UV stabilizers, that is why we use marine grade paint vs gelcoat on all of our restorations. Keep in mind most of our resto boats we found, were still around only because people were too lazy to take them to the dump. One boat actually came FROM the dump.

Gelcoat repairs would be the way to go if a repair area is small and/or folks have the right tools/materials/skills to effect the repair and match the current gelcoat color. Alan tells me that small areas can be brushed on, sanded and buffed. We've never tried it.

Racers can let us know if a gelcoat boat sails faster than a paint boat. In the meantime, we know for recreational sailing there is no difference.

Hi Clark! I appreciate very much all you do to archive Sunfish and other small boat information, and to provide so much help and advice to so many people here and elsewhere on the interweb. BUT, it is quite inaccurate to describe gelcoat as "thick paint".

Gelcoat most usually (but not always) is pigmented polyester or vinylester resin. Gelcoat does not "dry"; when a catalyst is added, the fluid gelcoat thermosets into a crosslinked polymer. Gelcoat is almost always pigmented to provide UV protection to the laminate it covers. Gelcoat sometimes contains additional additives designed to harden the cured polymer ("tooling gelcoat"). Gelcoat does not usually contain a wax additive, and so will not hard surface cure if applied and left exposed to air; this can complicate the process of re-coating a finished laminate with gelcoat (polyester/vinylester "laminating" resins usually do contain wax so they *will* hard surface cure when they thermoset).

Gelcoat is usually applied to the mold such that it will measure around 12-18 mils in thickness when the part is cured and released from the mold (1 mil=1/1000 of an inch). This is much thicker than most paint films (auto paint usually measures around 4-8 mils). The added thickness of gelcoat gives good UV and water-penetration protection to the underlying laminate, and allows for (careful and delicate!) fine sanding and polishing to restore color and gloss to weathered gelcoat.
See, I tricked you...I know that gelcoat is not "like a thick paint" but that is pretty much my limit of gelcoat knowledge. I knew that someone with useful knowledge of the gelcoat subject would come along and add that knowledge to the exchange.

Do you work with gelcoat?
I got curious about the thickness of the cutout piece.
Weighing the sample, it did not seem like it was solid or layered woven roving and resin.
It was suggested on another forum that it might be a 'coremat' type material. Using this in the math made sense.
I asked our section lab at work to take a look at it. It is s standard procedure of putting a material on edge, adding a potting compound, and grinding it smooth to reveal the items of interest. Here is a pic. I also reached out to LP to see if they had information to correct my understanding of the layup.


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The older boats are made by first spraying the gelcoat outer layer into a mold, which has a wax release. The gelcoat is used to protect the fiberglass matric and to make the boat's finish pretty. Then a layer of woven roving was laid in with polyester resin. Some areas like the keel are thicker.

The night crew would spray and fiberglass the mold, then day crew would pull the boats and clean/prep the molds for the night crew. In the early 70s 60 Sunfish a day were being made, plus a variety of other new AMF designs that entered production.

The newest Sunfish have some type of core, sandwiched by something. Not sure what it is but I hope it does not absorb water like a lot of balsa core boats do.

The hulls up through the mid 80s are very well built and very repairable. In 1989 Pearson dropped the weight 10 pounds and switched over to the rolled coamings. The few hulls we see that new have thinner gelcoat/fiberglass and are not as stout.
In post #1 I believe the photo shows a dry glass repair over a broken chine. Properly done the glass would be fully wetted and the base would have been well prepped by sanding. This illustrates how difficult this is inside a hull. Had the aluminum hull to deck molding been removed a proper job could be done and the deck joint sealed.

Two methods can be used to refinish a hull once the gel coat is damaged/gone. A new gel coat can be applied using available kits. I did this once with great success on a Laser. The other is using an underwater rated two part yacht finish such as AwlGrip. I have done our 58 foot ketch twice this way. Coating lasts around ten years in the Caribbean with care. Their latest offering is repairable. The finish is glassy, hard, perfect and as good as gel coat. If you are very skilled this can be DIY including roll and tip. If you are in doubt, hire an experienced pro.

For a quick OK job try Rustoleum water based two part epoxy. Dilute with more water and apply cool and shaded so it can flatten out. I rolled but an airless might do better. Tornado and sunfish. GO BLUE!


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