Gel Coat, why?

[sluissa]

As I was reading over yet another possible delamination or at best simply bad manufacture of a small area, I yet again wondered. Why do we put up with gel-coat? What's so great about it?

I've not dealt with gel-coat in any real regard. I don't even think my boat has gel-coat. So here's what I'm asking.

What are the up sides to gel-coat? I'll take anything, aesthetics, durability, strength, whatever.

What are the downsides to it? Again, anything, delamination, holes, brittleness, blisters, whatever.

Honestly, the whole thing seems to me, just a fancy finish(yes I know it usually goes on first in molds) builders put on new boats to impress buyers so that either A) they can charge more or B) they draw attention away from bad construction.

[David M]

A lot of the custom boats like Gunboat and FastCat are being done in LPU instead of gelcoat. I am thinking the same as you sluissa, what's the point of gelcoat? LPU paint is so much shinier, does not oxidize or have pores that hold grime. I am curious to hear some answers as well.

[phorvati]

Below waterline, gelcoat is there to seal water out. Polyster, vinylester or epoxy is porous and will absorb and soak in water causing blisters, cracking, and structural damage.
I learned a lot about the topic from David Pascoe, really really good articles. Yacht Survey Online: David Pascoe, Marine Surveyor

[David M]

I have always read different. Epoxy is watertight. Polyester resin and gelcoat can absorb water. I have seen lots of boats in yards that have severe blistering under the gelcoat because gelcoat is subject to water osmosis. The solution always seems to be rolling on some coats of an epoxy barrier coat like Interlux 3000 because it is watertight.

David

[44'cruisingcat]

Originally Posted by phorvati

Below waterline, gelcoat is there to seal water out. Polyster, vinylester or epoxy is porous and will absorb and soak in water causing blisters, cracking, and structural damage.
I learned a lot about the topic from David Pascoe, really really good articles. Yacht Survey Online: David Pascoe, Marine Surveyor

Epoxy isn't porous. If gelcoat is more waterproof why do so many builders seal their gelcoated boats with epoxy below the waterline?

[cal40john]

Gelcoat isn't as shiny, but it can be an 1/8" of an inch thick or more when put in the mold when the boat is made. My friend's boat got 25 years of buffing off the oxidized layers to shine it up over the years before getting down to print through.

Trying to paint gel coat on after a boat is built or to re-furbish, I believe makes things much more of a toss up. Painted on gel coat isn't as thick as when put on in the mold, and it's not self leveling, even with Durathane added.

John

[Joli]

Gotta put something there or the boat will look like raw glass. The choice is either in mold gel or after mold paint. If the mold is high quality you may as well just shoot the gel coat in, you can always paint it afterwords if you so choose.

[ssullivan]

Joli's right on that one.

Gel coat does 1 thing and 1 thing only: It makes boats look all sleek, shiny and sexy. That's all its for.

Take a good look at a lot of commerical fiberglass fishing boats where people aren't rich.

They are just fiberglass, sometimes with a coat of paint, sometimes not. It serves no purpose but to make the boat look good. This is one reason a gel coat void isn't a big deal.

It is also my understading (live David M) that gel coat is porous to some extent and that's how water can get to the first layers and cause blistering.

[Alan Wheeler]

Polyster, vinylester or epoxy is porous

Well not entirely true for the esters. In fact, if they were porous we probably would have the osmosis problem because the water would get out again. However, yes they are porous in a ruff sense of the word. Epoxy isn't porous. The reason for the difference between the two is the solvent and the fact the epoxy does not have solvent. Esters use styrene as their solvent. As the chains build and the styrene evaporates, small cellular structures are created. With Epoxy, there is not solvent and so 100% of the epoxy becomes hard and solid.
Never put solvent into epoxy resin to aid flow. It causes it to have that porous problem when it is hard. In fact, Evadure used to be the real bees knees for hardening and waterproofing timber hulls. But Epiglass now suggest not to use evadure in that manner any more, because it allows the timber to still take up water. You may not get rot perhaps, the hulls weight can greatly increase due to water absorbtion.

[phorvati]

Thanks Alan and 44'cruisingcat for the corection, I jumped the gun on the epoxy.

As far as those fishing boats with just some bottom paint. I can say first hand that they weigh much more at haulout at the end of the season then at launch in the spring. They also use more and more gas as the summer goes on because they absorb more and more water, at least the ones built with polyester/fiberglass or vynilester/fiberglass. So below waterline, gelcoat will help prevent water penetration into ester based hull.
And this comming from a person that put an epoxy barrier coat on the bottom over the gelcoat? I did it because the gelcoat was cracking. I was not sure if the cracks were deep or not, but for a production boat with polyester hull and balsa core, I thought epoxy barrier coat can only help.
Petar

[Roy M]

I've been using epoxy for over thirty years. First, because polyesters don't seem to want to stick to themselves. Drop a wrench on a polyester/fiberglass deck and you don't see anything at first. A couple of weeks later, you might notice a slight bubble in the deck. A couple months later and it has grown. A bit more time and the glass breaks, letting water in, with resulting greater damage. Epoxy with/without fiberglass simply star-fractures but doesn't release from the substrate like polyesters. Gel coat is merely a thicker coat of polyester resin, mixed with some wax, if it is not cast in a mold, to make it harden and not remain sticky. It has no significant strength, at least compared to epoxy. The only advantage to the gel coat, besides lower cost, is that it can have pigments to slow down ultraviolet destruction. The gel coat gets "chalky" with time and needs to be waxed to protect the underlying polyester. LPU paint is a superior finish for both gel coat and epoxy finished boats. You can even get clear LPU to coat an epoxy/fiberglass hull of nice looking wood. It doesn't last as long as an opaque pigment, but some folks like rolling rocks uphill. I have used epoxy underwater (with graphite powder for abrasion resistance) for thirty-plus years. I have built epoxy plywood fuel tanks for diesel that have lasted the same. Recently, I bored a hole in one to install a fuel tank sender. The plywood core smelled like brand new plywood. No penetration, period. The same for installing transducers in an epoxy/plywood hull. It's difficult to counter evidence like that, don't you think?

[ssullivan]

Huh... we are left without a good answer.

Once concensus is that gelcoat is porous enough to allow water through (as in the case where we find a boat with an intact gel coat layer that has blisters). Seems to be obviously proven by the fact that boats get blisters below the gelcoat.

Now also, we have Petar's first hand experience with fishing vessels that aren't gelcoated getting heavier and heavier as they apparently absorb water.

If we take these two facts together, it would seem that *all* sailboats with gelcoat made of esters should in fact get heavier and heavier as well with time in the water.

I'm not aware of the being true. So... this isn't all adding up.

[44'cruisingcat]

The answer to the original question is simply - cost. If you have made female moulds for a production boat, the most cost-effective way to get a nice finish is to spray gel-coat inside the moulds before laying up the glass. You simply pop the boat out of the mould and it's already painted. No need for all the preperation, primers etc.

[chris_gee]

As I understand the first fibreglass boats were polyesters. These are fairly permeable to water. Vinylesters are slightly permeable, and epoxy virtually impermeable.
Osmosis in the general sense is the movement of fluid from a dilute solution to a more concentrated one.
The problem is that the barrier allows small molecules like water through, but not larger ones.
The problem with osmosis in boats appears to be that there are soluble organic compounds that were used to coat particularly the strands of chopped mat. It seems these are the culprit and they may not be there if a chopper gun was used because they werent required to loosely hold the strands together. They may also not be in woven mat.
As some water comes in these dissolve to form a concentrated solution which then attracts further moisture. Pockets of this presumably concentrate in any small void and expand eventually rupturing the surround.
Simply drying the hull for sufficient time will remove the water but not the compounds in solution. Any expanded voids will remain even if obvious holes are ground and filled. They will to some extent be a source of weakness.
The process will continue on re-immersion with further water ingress. This will be minimal with an epoxy barrier, presumably slightly more with a thinner expoxy paint, more with vinylester and more still with polyester.
Views on osmosis treatment and whether it was simply cosmetic and of no consequence eg Casey have changed over the years particularly because it has recurred quickly after expensive treatment.
Current thinking seems to be that full treatment involves peeling the gel coat, and repeated washing to try to remove the solutes, and then drying and recoating with a better barrier coat, after any restoration of thickness required. This is a lengthy and not cheap business.
To go back to the original question. "Gelcoat" as such is not required strictly but some coating that fairs, waterproofs and is decorative is. The alternatives of barrier coats and paint fulfill the same function but may be thinner and depending on what resin was used for the gelcoat may be more impervious to water.
This is a lay and non technical distillation of what I have read.
It may be that boats differ in their tendency to osmosis depending on how they were laid up and the use or not of chopped mat binders and different resins.
Conceivably a boat with relatively permeable polyester but no water soluble solutes could take up water but not form osmosis. Equally a boat taken from long term salt water to fresh might assuming salt can pass through the gelcoat being smaller than the other larger chemicals.
The fact that boats may take up water may explain that the common observation that their weights on travellifts are often greater than expected beyond the increase due to fuel water and stores etc .

[jim lee]

Actually gelcoat is not usually the same material as laminating resin. One is typically an Iso-something and the other is an Ortho-something. Anyway what it boils down to is the gelcoat is a tougher material giving a better protective shell to your laminate.

Gelcoat does seal out much of the water. Almost all of it if you compare it to not using any barrier. Think about it, a bare glass hull would have a bizillian fibers right there at the surface going; "Come here let me absorb you into my wonderfully wicking glass matrix". I'm not surprised the raw glass fishing boats soak up a bunch.

Glecoat is also used as a release agent for getting parts out of molds. The molds are gelcoated to seal them up so resin doesn't get into the mold laminate and lock in the new part. Same with the part itself. Spraying a coating of Gelcoat into a gelcoated/waxed mold does a great job of sealing up everything and leaving two tough surfaces that can be pulled apart later.

We had a mold where we'd sanded thought the gelcoat into the laminate. My buddy wasn't thinking and tried to pull a part from the mold. (Before repairing the gelcoat) The part's gelcoat got into the mold laminate.. What a disaster! I just got from spending hours doing mold repair from that little boo boo.

I've got gelcoat on my mind..

-jim lee